These things don't get huge. A 5" pleco would probably be fine in a 15 gallon for most of his life, not that he wouldn't be happier in the bigger tank. The bigger tank WOULD allow her to get more plecos though . If you have access to them, these guys LOVE live blackworms.
Hey Kryten. I got 2 of these little beauties. Lucky me i got them for £20 each.
I've found them to be extremely peaceful, very active (even during daytime), easy to feed (courgette, sweet potato, cucumber, lettuce, different algae wafers, blooodworm and raw prawn).
I'm from Ipswich and I got mine from a place on the A140 called Stonham Barns.
Ouch, I'm glad they aren't so pricey on this side of the pond. I'm getting a group of 4 for $60 (plus shipping, ~$70 total). That converts to roughly £18...