I just made a ganche (hasn't set up yet) and used bittersweet chocolate. 
It's really bitter tasting! I'm using devils food chocolate cake and white BC icing with MMF accents.
Can I add sugar to the ganache or will the sweetness from the other parts of the cake even it out??
I don't want everyong thinking the cake was good except for the filling!! 
I've done bittersweet ganache in chocolate cake several times and it's always good. I like my fillings (chocolate or fruit) to be not very sweet because I think it balances out the rest of the cake. Especially if you're using a super sweet buttercream. I've done chocolate cake, bittersweet ganache and white chocolate IMBC too (IMBC isn't a super sweet frosting) and it's always a big hit. HTH
I agree with the filling not being too sweet - I prefer it balanced also. If you are still concerned - why not do a layer of buttercream then a layer of ganache for the filling? that way you're adding sweetness where you want it.
It will be perfect with semi-sweet! And oh, by the way, there are NO stupid questions!
IMHO I think semi sweet or bittersweet make perfect ganache. If you used milk chocolate it would be too sweet and hershey kiss-ish tasting 
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