If you can, put the guppies in a 10 gallon with the baby Betta and hope for the best. If it didn't work out you could divide the second 10 gallon. I've kept a guppies in a 5 gallon with no problems. While not a schooling fish, guppies do like the company of their own. Get at least two more; odd numbers work best.
At least you didn't get pond snails as your hitchhiker! Guppies are a lot more fun.