It depends on how your Betta will react. Mine like to kill any other fish I put in their tank, juvie or adult it doesn't matter to them, this is their territory. So I'd definitely suggest to at least put the Betta in last after it's all Cycled and stuff. And yes your Betta will eat the fry, I don't know if he'll eat all of them but most of them for sure and as often as your females will drop their fry.
When you notice the fry disappearing, I would cut back on his food a little bit, only a few pellets (I would definitely do pellets and you can do flakes for the guppies). I don't know how much fry are nutritional or anything I would guess lots of protein.
So I guess it comes down to; just be careful, if you get a longer tailed boy it's going to not eat as many fry because he can't catch them as well with the tail dragging him along but if you do want him to eat the fry, get a shorter finned EE boy; PKEE. Or you can just keep them separate, so it's a matter of choice here and what you want to see. Do you want to control the guppy population? If yes, then put the Betta in, if not then don't put him in