Civilizations hire mercenaries

One thing I noticed as Japan is forget about invading the Khmer with your two starting samurai, they'll "hire" a whole bunch of mercenaries right before. Same thing with Indonesia.
However, Indonesia has a problem. Since they are islands, all their mercenaries are hired in their capital, which means Jakarta and Singapore are easy to take. So there you have it, I took Jakarta with one samurai and a galley early on in the game because the mercenaries Indonesia hired only stayed on one island. Interesting observation I had.
How is it actually calculated when a civilization should "hire" mercenaries?

I would need to look that up. It's not really worth it though, because the current mercenary system will soon be gone.

It'll have a new one , or no mercenaries no matter what ?

Not in the form as we currently know it. The ability to hurry production with gold will be split into units and buildings, and there will be a relatively early civic that allows to hurry units with gold.

What will Carthage's UP be then?

Halved price for hurrying units.

May I suggest Fanaticism? I almost never use it, and it could sure use a buff.