Supply, Allies and what happens if an alliance is broken

[Pokke]

It's my first game. I conquered most of Italy and am now focusing on the North.

I have Boii as allies for Rome, since early beginnings.
To show my empathic nature I wanted to help them out in their war with some tribe -which name I do not recall. So Roman legions moved through Boii lands into the North and switfly killed the 'bad tribe' army. Somehow, dealing with these, also gave me their land (which I did not want). Worse. Despite being allied with the Boii my supply network does only seem to go inside Boii land but not past them?

So now I'm wondering if I should a) simply conquer the Boii or b) wait till they ask to be part of the great Roman Empire or c) try to figure out how to make them a client state.

Option a. (conquer them) What will this do with my other allies? Will relationships drop if the rest of the world sees that an alliance with me is simply discarded?
Option b. (they ask...) Does this only happen with client states or is the trigger when they're on all sides surrounded by me?
Option c. (client state) Despite being allies and having a good relation (+63) they seem unwilling to become a client state. I'm not sure what can improve that one?

Thanks.