The pre-Marian Roman army was a militia recruited from all Roman citizens who met certain property requirements. Legions would be recruited for each specific campaign and then disbanded afterwards, there'd be no standing permanent army.

So each legion would be comprised of a mix of veterans and green soldiers joined up in a new formation. Depending on how fast their general moved them, they may have had little time to drill as a legion - though this could be a literary topos to excuse defeats.

As it was a militia, each soldier was responsible for supplying his own equipment - all based on a specific pattern (e.g. gladius, scutum, pila, etc...) With things like tunics you're probably looking at tremendous variation as there's no 'uniform', just what that soldier is wearing. When you get into the Late Republic and Imperial times there's a big debate about specific tunic colours.

So what you see could vary depending on if the legion's had time to drill and settle in or was it rushed straight from the Campus Martius to battle. A veteran legion would march with confidence and be precise in their movements while a green legion may be hesitant and awkward reflecting the lack of trust and cohesion built up in the unit at this point. Their equipment would be very similar, probably identical, but their tunics would be a kaleidoscope of various colours.

p.s. I think you're referring to Romani hastati as hastae is 'spears'.

Just a quick note, the plural of Hastatus is still Hastati, not hastae, which means 'spears'.

Essentially, A pre-Marian legion was formed in several parts:

The first battle line was composed of the hastati (roughly translated as "spear"), they were the first to meet the enemy man to man and often used poorer equipment than the next two infantry maniples. The hastati were screened by the Velites, who were the poorest in the legion and acted as skirmishers who threw javelins at the enemy before the hastati moved in. The hastati often wore leather cuirasses or small bronze plates.

For weaponry the manipular legions used large oval shields called "scutums" and a shortsword, which they adopted from the Samnites, an italian tribe who fought with Rome for decades.

The principes (roughly translated as "first") were next, they were richer, more experienced men who helped when the situation for the hastati was becoming troublesome. Later they wore chainmail (adopted from the celts) and the same weapons as the hastati.

The principes and hastati also used the pilum, a javelin with a deformed tip that made it harder for the enemy to remove. They most likely started using these from the celtiberian tribes of Iberia, modern day Spain.

The next body were called Triarii, they were the most veteran of the infantry who served in previous wars and were richer than the Principes and Hastati. They used the old hoplite style of fighting and moved in when the situation was becoming dire (there is actually a phrase called "going to the triarii" which means that almost everything has been tried and failed). Behind the triarii were the Rorarii, who were the reserves for the reserves. The triarii first had the equipment that was similar to the Greek hoplites, but later traded the hoplon shield for the scutum.

The cavalry of the army was mostly composed of Rome's allies (called "socii" in latin) and the equites, who were composed of the richest of Rome, there were around 300 of these. They were not as great at the 'traditional' duties of cavalry (chasing down skirmishers or lightly armoured troops due to their rather heavy equipment - they could afford the equipment) but were definitely better at being shock cavalry.

Here is a good picture of how a manipular legion would have been formed

All the answers so far have only mentioned Roman troops. Yes, Roman legions were, prior to the abolition of the property requirement, stationed in three lines, etc. The hastati and principes actually were not divided quite as simply as is often said--the principes were often hastati that had been promoted, but one could also be stationed in the second line with one of the principes despite having no prior experience by belonging to the third property class (the hastati were mainly made up of fourth-class citizens).

But Roman armies of the middle Republic and earlier were not primarily composed of legions, not during large campaigns. The city, even with its multiple colonies in Latium and elsewhere, simply did not have the manpower to raise armies of the size which we see fielded against the Carthaginians and Macedonians and certainly not as many armies as existed or as often. Roman armies of the period were composed to a great part of allied contingents--about half the troops at Cannae were Italian allies, with only the right flank being held by Romans. We do not know how these allies were equipped or arrayed in battle. It seems unlikely that they were all drawn up like Roman troops, but to what degree they fought according to their own customs is hard to say. It's possible, for example, that Greek allies fought according to their normal customs. It also depends on the relationship of the allies, as during the middle Republic Rome had a very complicated system of alliances with its subject cities, which usually were drawn up for each city individually. Some retained more freedom than others, and for quite a few the Romans hardly interfered at all. Both Polybius and Livy speak of allied troops being present at a number of battles in large numbers, but none speak of what they looked like. The only real indication we have is when allied states provided troops, such as the various Greek allies fighting in the Macedonian Wars (which saved the right flank at Cynoscephalae when their phalanx held firm against the Macedonian charge) or coningents from allied states such as the Numidians. These troops fought according to their ancestral customs, but they weren't allied the way the cities of Italy were, but truly independent states.

It really, realy depends on the year and who is in charge.

visually, you can expect a LOT of feathers and crests, and you're goign to see helmets by the thousand, along with shields. Beyond that, tunics and swords (of average length for the day.)

Everything else is completely at random-some will wear pectorales, some musculta, some will have chain armor, some no armor, some just a greave, some two, some none, ect.

You wouldn't see a lot of dyed clothing-it's expensive. Why risk it in battle?

Int erms of how they moved, J.E. lendon provides a good argument that the first two lines of troops didn't even HAVE a regular formation. Essentially, they just occupied the required area in clumps around their banners, with one, regular line of men with spears behind.

As the period wears on-in particular, towards the end of the second punic war-soldiers get forced to serve for loooooooooong periods, and the armies get a lot more experienced, so they'd seem smoother in their movements.

So what would you see?

A big body of medium infantry with shields, shiny helmets. It would extend too far back for you to see how deep they were formed, and the flanks might be made up entirely of your neighbors, a people armed just like the romans, or somebody you've never heard of , much less seen.

On flat ground, you'd also see a cloud of light infantry.