..pleco, you mean? Or Hypostomus plecostomu?
Trust me on this one: you really do not want a H. plecostomu because they should have bigger tanks than either of yours.
The 28 gallon is a 3 fter? Then how about..
* 1 dwarf gourami (or a lace gourami or a three-spot gourami if being moved to the larger tank quickly and the larger tank is a 4fter)
* 15 serpae tetra or any other 5cm tetras of one species
* 1m 2f Apistogramma of one species, such as Apisto. cacatuoides or A. agassizii, or 1m 1f Laetacara dorsigera
* 1m 2f (young) bristlenoses, which you would need to move to the 55 gallon as a 3fter would be too small for 3 adult fish
You could then move all of these into the 55 gallon (3ft or 4ft?) to get this stocking:
* 1 dwarf gourami or lace gourami or any colour variant of a three-spot gourami
* 25-30 tetra of one species (30 if 4ft)
* 1m 3f Apistos (1m 4f if a 4ft) or 1m 1f Laetacara dorsigera
* 1m 1f Bolivian rams or keyholes if a 4ft
* 1m 2f (young) bristlenoses
Neither of those lists are such high stocking that you wouldn't be able to add anything else.. so leave *some* room for more.