I would still get rid of the pleco though, once they get bigger they become really lazy and don't do much more than poop. So it will only cause problems for your tank. If you really want a plec, a bristlenose plec would be perfect for the 38 gallon.
Some stocking suggestions:
Keep the betta in the 3 gallon by himself. Move the tetras, swordtail, cory's and glowfish to the 38 gallon. Get some more cory cats, and maybe some more glowfish or swordtails, whichever you like better. The Rainbow shark can go into the 75 gallon, and you can pick other fish that will work with it. Just make sure the 38 gallon is fully cycled before you move everyone.
If you really want to keep the swordtail fry, you can get a breeder net for the 38 gallon, or you can move the betta into the 38 and give the fry the 3 gallon.