Wait until you've got a bigger tank. Then get the fish. Far more fair on the fish and definitely the right way round to do things.
What species of Pleco? If it's a Common or Sailfin type you'll be needing to aim at a 100 gallon. If it's a smaller species like a Bristlenose aim for at least a 20g.
Clown Loaches, indeed any Loach, I wouldn't recommend with Bettas. Loaches can be very active and that can really bother slow-swimming fish such as Bettas. Clown Loaches also need to be in groups of at least 5 or more, and as they eventually get massive, need at least a 6ft tank.