It is possible to unnaturally stunt a fish's growth by keeping it in a tank too small for it- that's where the whole "a fish will only grow to the size of it's tank" myth came from in the first place.
It's unhealthy, however, and cruel.
That being said, clown loaches do grow somewhat slowly. My clown loaches are about 8 years old and about 6" ATM- they're about to "upgrade" from my 46gal into a 90gal tank.
Your rainbows are already too large for the tank- especially as they need to be kept in schools and schools need swimming room.
If your pleco is a "common" pleco then they actually can grow well over 14"- and plecos grow rather quickly.
So I personally believe you have some serious problems with your current stocking, and a conscientious fishkeeper would want what's best for their fish and address the issues sooner rather than later.
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