Establishing a tank that requires no filtration is finicky, at least if you want fish. Did you have any ideas as to what you would want in it?
A rescape of an already established 55 would be interesting, to say the least, if you want to keep your fish in there. Personally, if I was to plan on changing out substrate and re-scaping a full tank, I would remove all of the fish and essentially start the tank anew, which would include re cycling the system. Also, if your pleco is large (1.5yrs so probably good sized), he/she may limit some plant choices. Plecs have been known to graze/eat/remove some broad leaves from certain plants.