Bettas in a large tank is not a problem. They love all the space they can get. There are a few random males that can get stressed from guarding such a large territory but it is rare. Be aware that guppies are not always a good choice for bettas (male or female). Female bettas are every bit as aggressive as their male counterparts and a lot of the time can actually be more aggressive. If you keep multiple female bettas, the number needs to be 5 or more or you run the very high risk of one girl taking the brunt of all the aggression. Think of it this way -- the more females the less one particular fish gets beat up. To be successful, you need a lot of hiding places and a lot of plants or other "line of sight" breakers. That way if one goes to chasing then they get "distracted" once they lose the one they are chasing and rarely every "hunt" them down.
Tankmates for bettas need to be chosen with care since they are either going to be the chased or the chaser. Some fish can nip at the bettas fins and stress the bettas out. Other times the bettas will chase and hurt (worst case scenario - kill) the other fish. Females are notorious for ganging up on other fish (another reason for the numbers of female bettas to be above 3 at all costs) so you have to watch for that as well. The pleco will be perfectly fine as I personally have never had a female that was overly aggressive to something like a pleco or cory.
Guppies could work and have worked with other people's tanks so it is not impossible but just be aware that it just as well might not work like you plan.
