No betta in freshwater. Also, a GSP will kill it. Just about anything without long, flowing fins would be fine with a betta in a ten.
A GSP needs at least a 29gal by himself. They are very aggressive. They do need brackish water that will eventually increase to saltwater.You'll need to get marine salt and a hydrometer. Start him out at 1.004. Acclimate him slowly. Of course, the tank would already be cycled. They will only eat frozen and live foods, mostly bloodworms and pond snails (set up a small breeding tank) If they don't get something to wear down their beaks, you'll need to trim their teeth. HTH
If you can't give the GSP what it needs, take him back. If you really want one, then you can get another one when you're ready.