well, move the plec ASAP because otherwise he will stop growing and die, the loaches are slow growers, but it is best to remove them.
A 2+ gallon tank is a tank size above 2 gallons. That is all the betta needs (and a heater unless room temp is warm where you are), the reason it should be alone is because it is aggressive, and will try and kill your guppies. And the guppies will nip the betta. If this hasn't happened yet, it is better to not risk it.