- At what age can/should we introduce sign language?

I read that you should introduce the signs around 6-7 months - you can, of
course, start whenever. With consistent signing, most babies begin to sign
around 7-9 months. "Sign with Your Baby" suggested starting with "more,"
"nurse," and . . something else. "Help?" Just because those are very
useful signs.
We are just starting to sign with DS 3.5 months, so we'll be in the habit.

- What kinda sign language are we to use?

There are "baby signs" but we found it easier to use American Sign Language.
The baby will adapt the signs to his ability, and/or make up his own. Just
as long as you understand each other - that's the point!

- any book recommendations?

I'd just buy a few basic ASL dictionaries to have at hand. But I know other
people do videos, classes, etc.

- If I make up some signs of y own, would this confusethe baby? (I know
it sounds stupid, but I was told that it would. Which I really can't see
as logical at all)
I see absolutely no reason this would be true.

- Does the fact that our baby is gonna be raised tripple or quatro
lingual mean that sign language will be harder for the baby to learn? (I
mean it is an added language)

I don't think so. Signing is really useful for the pre-verbal stage. Most
babes gradually stop signing as their verbal communication improves.

Hannah