I actually prefer buckets, but my tank is only 30 gallons. A friend of mine has a python, but it seems to waste a lot of water and involve either a) a partner, or b) a lot of running around.
And it sounds to me like as long as the media you are putting in the new tank is established in the old tank, the pleco should be fine. It sounds like your plan for testing/changing is a good one. After all, one plec in a 55 isn't much of a bioload to begin with.