Bonus question. I once read (can't remember where) that often the crucifixions were done on single vertical poles.

Is this true, and if so, was that a more normally used method by which they crucified people?

There's no standard definition of what a cross actually consists of in Roman literature, and there's no standard definition of what a crucifixion consists of either. A 'crucifixion' is just a vague conglomeration of concepts around torture and hanging something up on something (crux is used by Pliny to denote hanging up some vines).

There's a number of words used to describe the processes of crucifixion (crux, patibulum, furca, stipes, and arbor infelix) but there's no standard definition of what they mean or how they were used. You suspended the victim to a crux which is some sort of wooden device, and sometimes you attached them to a pole. The patibulum is simply just a pole or a beam, and can be some sort of crossbar but there's no particular way it has to be fixed. Being attached to a pole was a crucifixion, Crucifixions also involved rectal impaling, stabbing people through their genitals into the ground, and attaching people's genitals to tree branches, so it's a bit difficult to nail (ha!) one definition down without it getting up and wandering off ^1 to have chat with some other definitions.

In answer to /u/Bobarhino , quite a lot of executions were done like this, but nobody really describes the process of crucifixion enough to come to a final answer. Pole suspensions (crucifixions) could be done on the living or the dead, but there's no way to say whether it was more common or not, simply because nobody bothered to explain what they were talking about explicitly.

^1. Probably slowly.

Also, which method (the cross or the t ) for crucifixion would be the most preferred? As in, which type would be more painful, or prolong suffering?

Crucifixion wasn't meant as a merciful death, so I imagine the societies that practiced it would prefer the most torturous methods to the victim, correct?

Which method would achieve that?

There's a few different reasons and I've seen a number of books address this issue - the problem being that they were all on the illuminati/freemasonry conspiracy end of things. First - like has already been said, Romans used a lot of different shapes. Secondly, various crosses were already in existence and may have been co-opted into Christianity. Remember early Christians didn't focus on the symbol of the cross, they used the fish.

The Egyptian cross (with the little circle on the top), the Tau (a T, not a cross) and others have all been exhaustively written about, and I honestly don't have a clue as to the validity of all these connections, except to say that like language, symbols are borrowed and given new meanings all the time and why should this be any different.

Basically, it just happened, probably through borrowing, but any attempt to determine exactly how is going to take you into a world of conspiracies ala Dan Brown.

edit: the Egyptian cross is called an ankh, https://en.wikipedia.org/wiki/Ankh - this article mentions that Coptic Christians preserved the Ankh in their Christian symbolism.