The best options for mobile or low-spec devices is Vorbis for music and FADPCM for effects.
You’ll need to do two separate banks if you want to support both formats.
There’s no restriction on the number of sounds that can be in a bank.
Our Sound Bank tools can accept any format our runtime can playback. But you’ve already identified that giving it compressed formats will result in decompressions/recompression and signal degradation. There’s no concept of simply re-packing pre-compressed files, it’s infeasible for reasons I won’t go into here.