You can always put them in there but there is no gaurantee that there will be no problems. Guppies have been known to be fin nippers on Betta fins and the amount of water is not going to make a difference and if you have Guppy males with fancy tails, the Bettas have been known to go after them as if they were another Betta. I cannot keep you from it, no one can but we have advised you not to do it because it could be harmful for either of the fish. It does remain your choice, but we want you to know there are risks to be faced if you do.
You will have to keep a close eye on the health of all the fish because if there is nipping of any kind on any of the fish's part, there is going to be a good chance of infection setting in. Also there are medications that are appropriate for Guppies but you cannot use them on Bettas so if you need to medicate your Guppies now the Betta will need to be removed to another tank or the sick Guppies will.
It really is just a better idea to keep them seperated.
Rose
