what synth circuits are you wanting to explore? a lot of the classics will respond just fine to a CV of 0-5v. some might want to see a bipolar CV (say -5 to +5v), some will track up to +10v. unfortunately, there doesn't really seem to be a real standard. starting with the 0-5v range of bela not only puts you rather firmly in the safe zone, it should also get you decent results already.
if you do happen to come across circuits with different requirements, you can start to convert voltages, but if it ain't broke - don't fix it.
personally, i'd run the synth circuits on their own power supply and just put in a common ground connection.
most synth circuits require a bipolar voltage anyway (+/- 12v is pretty common), and that would be hard to get from Bela.