yes, a betta could eat guppy fry if they were still very young, but it would be inadvisable to expect one betta to eat what sounds like a very large amount of guppies in a very short time. I fed one of my bettas live endler fry which are very similar to guppy fry, but at the age that a betta could eat the babies, it might have a hard time catching them. My betta ate from my hands so I would catch and hand feed him one baby and no more at a meal. One time i released the fry too soon and the betta was never successful in catching it, though he tried.
After a short time the fry would be too large for a betta to eat and if it did catch one, you'd have an ugly situation on your hands. they can really rip things to shreds when they want and probably wouldn't kill and eat one at a time so much as attack and bite any that were slow on the uptake. And then you'd just have a lot of injured guppies regardless of age.
Your best bet would be to find a store that will take your excess fish. Or if you don't mind euthaniasia, there are a number of ways. A very effective method for small tropical fish is dropping them in water that is nearly frozen (in other words, has been in the freezer until it develops a skin of ice or becomes slushy). They'll die of shock almost instantly.