1)what is the result of a bloobath(12 points of barbarian against 3 points of Roman in a fort -so 6 points-) Do you lose 3 or 6?
2)If as A Roman I play a foederati and loose a battle againt a barbarian attacked by a barbarian... do I lose a chit?
3) The barbarian is playing a foederati after taking a Roman command chit... but the Roman plays agentes....so the barbarian is giving back the command chit to the roman or do you put it away?
The Agentes cancels the foederati, so the chit goes to the Roman player as normal.
4) stacking : in the rules : 3 units max.... but static units do not count so it is 3 unites + a base?
Yes, though it could also be 3 units + (base or militia) + leader.
5) terrain : a unit in a city behind a river is multiplied by 3 or 4?
I'm 99% sure that is all correct.