> Unless I'm wrong, southern or far southern India was a huge and important exporter of luxury products during roman imperial era in the west, being most goods exported expensive products of animal and vegetal origin (like spices); rather than valuable metals, jewels or manufactured crafts.

You are correct

> I tried to play a miniature war game called The Bellis Antiquitatis and after getting interested in the ancient Tamil or southern indian army lists, I got surprised for the option of including jungle hunter-gatherer units (adivasi) in the army list. Also, the aesthetics of professional miniature companies usually represent very light armored warriors or just covered with kilts or loincloths. I know that in warmer climates people get some pieces of clothing off, but seemed like specially vulnerable armies to stand before some possible conqueror or raiders like southern arabian kingdoms, Kushan, parthian or roman fleets.

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lot of that is just because British historians really didn't keep great records of this type of stuff so they use stereotypes. You can see in Temple art and statues the the Tamil armys were quite well armed and protected, these are the people who created Wootz steel (Damascus steel is just imported wootz steel). The Chola navy at its height was the largest navy in Asia and the foremost naval power (Kind of like the Vikings of India but they didn't pillage and rape everywhere they went)

> Did they need such defenses against sea invaders or prevailed a sort of "cosmopolitan trade points" among continental powers??

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They definitely needed defenses and they had it, the Tamil Kingdoms were the only power in India to not be fully conquered by a North Indian power ever, until the Brits arrived they were not conquered by a non-south Indian power, They had a well trained and powerful army stocked with war elephants, infantry and what cavalry they could obtain. Tamil archers were also quite powerful

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> Why during the Imperial Roman Era, Southern India and Ceylon kept more tribalistic and small political unities than the north,

They did not? Don't think the adivasis were the only people in the area lol Tamil Kingdoms were urbanized since before Rome was a thing, you can find imperial Pallva and pandya coins in Egypt, they were long trade links with the world which fueled many wealthy kingdoms with 3 especially powerful and rich ones (Pandyas, Cholas and Cheras) being the top of the many kingdoms at the time. They were not small political unities by any measure esepcially when you take in compared to similarly sized Greece or England the Tamil kingdoms had quite a high population. They were also surrounded to the north by the Kannidigas and Telugus who were also a bunch of urbanized, rich and powerful kingdoms.

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>Were regional Tamil kings or population relatively wealthy for that era??

Extremely wealthy, these were some of the richest kingdoms in the old world especially for the size all the south Indian kingdoms were, Marco Polo himself remarked it was on of the richest regions on earth.