A red tailed shark will not out grow a 55 gallon, already talked to an expert. (yes, i know they lie quite often at aquarium shops) and yes, depending on the kind, a pleco "may outgrow an aquarium.Pure ammonia will cycle a tank faster then it will with fish in it. If your planning on cycling with fish your going to have to do frequent water changes to keep the fish alive disrupting the cycling process.
Can you do it with fish?
Yes, you can cycle a tank with fish.
Are there other options?
Yes, as I have mentioned, cycling with ammonia without fish is fish friendly and a better solution.
If you want more on doing a "fishless cycle" (this is the method with pure ammonia) search these forums, google fishless cycle or ask questions.
Finally, the red tailed shark and common plecos will for sure get to large for a 55 gallon.
I agree, bala sharks need a 125 gallon tank preferably for one or two, many people put this fish in too small of a tank. Not to mention they need schools.and for the record liveaquaria isnt always right. i just went there and started looking at bala sharks. it says 70 gallon tank minimum best in groups 3 or more. if you keep 3 in a 75 gallon tank they will grow about a foot each. this will not work, and pref. balas should be kept 6+ which would require a large tank.