They can get pretty big, about a foot and a half I think. I'm not sure how fast they grow, but I think I'd want something larger than a 50. For the pleco too ...
Consider it this way A 55gal is only 12 inches wide. Both the pleco and the BGK get to be larger than 12 inches, so they would have no room to turn around at all. IMO the absolute minimum would be a 75 if you cant afford anything larger, but a 90gal would be even better! :thm:
To answer your question though yes that 55 will be too small for both fish eventually, but rather start out with the right size tank first than keep upgrading...save money that way and will keep your fish happy