Please listen to us. We are just trying to help you so that you can have a healthy tank. Yes, bettas can be kept with some other fish, but that is in a larger tank. Also, both the fish you have are incompatable with bettas. Male guppies, with their fancy fins, are often a spark for betta aggression. Goldfish are cold water fish and bettas are tropical fish so that is a BAD mix. Also, like has been said goldfish are huge waste producers. I honestly don't think that you will get a 2.5 gal to build up enough benificial bacteria to keep water parameters good. Also, your goldfish will grow fast provided it doesn't die. It would be better to just bring them back now rather than having to get rid of a dead guppy, and upgrade to literately a small pond for your goldfish in the future.
So please take our advice, that tank is now not a suitable environment for any of those fish. It would be much better to act now than to just say "it is what it is" and end up with dead fish later.