[quote=Ink Blood]
For the infantry training skills like sniper training and whatnot, do I have to buy the skill for how many snipers I want or just once to have snipers?
[/quote]

If you are referring to custom units then what you basically do is design your own unit, so if you add sniper training to them, then each unit you buy will have sniper training.

Basically you could make a custom units like this:
Mooks A:
Basic Blasters
Heavy Weapons

^ Those units are heavy weapons trained

Mooks B:
Basic Blasters
Heavy Weapons
Sniper Training

^ and those can both use heavy weapons and sniper training