You may have a bit of trouble with a betta in a Bio-orb. Sorry to say that sometimes they do not have enough surface for them. Bettas are surface dwelling fish due to the need to breathe air and the surface of a Bio-orb is very small and generally very active current wise. Sometimes the bettas are kind of plummeted about and with the finnage they carry it is like a sailboat trying to stay afloat. So while I am not saying not to do it I would have a back up plan in force in case your betta does not tolerate this well. Some do and some do not but if he is unable to navigate well to the surface he will have to be removed or he will drown in there.

Sounds like you are going to have one super quality betta and it would be nice to see some pictures of him when you have all established. Congratulations on the find.

The tank mates that were mentioned before are pretty much accurate and most would make good fish for that size bio-orb but probably not more than one type of them. Loaches or cories would be my choice for that size as they would have the strength for the current and provide an all over tank activity that would make it interesting. But the Dwarf or Chain Loaches would be the only ones appropriate for this size orb.

Rose