Hey,
Yep, Clown Loaches are nocturnal in the wild. This may mean that in captivity they might be more active at night. Also, it might be a good idea to feed them at night as well, because they are more alert.
Clown Loaches need to be in groups of 4-6. They can grow to about 12 inches, so you need a much larger tank for these Loaches.
BN Plecos only get around 4 inches, so they're perfectly fine for your tank. Keep in mind they are quite messy and create a lot of waste, but it's fine as long as you don't overstock the tank.
They grow pretty slowly. I'd say about 1-2 inches by the first year.
Hope this helps!