I think you might be able to get a hundred neons in a 55. I think I might be pushing 50 fish in my tetra tank. I have mixed in there cardinal tetras, rummy nose, lemon tetras, HY511 tetras, green fire tetras, harlequin rasboras, beckford's pencils, marble hachet's, coolie loaches, chocolate gouramis and a couple melinii corys.
I would suggest otos rather than plecos since almost any larger fish will munch on neons. I use 5 1/2 and ten gallon tanks as Q-tanks since you don't want to fight an ich infestation in a show tank of tetras. I treat all new tetras with one dose of Quick Cure at a rate of two drops per 5 gallons when I first get them and hold them for at least two weeks before introducing them to the main tank. The tetras seem to tolerate this low dose.
I really like HY511 tetras they look like bleeding hearts without the heart but grow only to the size of black phantoms and don't get huge like bleeding hearts.