He only looks just a little underweight at most to me, like you said maybe a bit skinny. Is he eating well? Poop looks ok. Does it eat yet? Did you ask the shop what it was eating & get that food? Sometimes it takes some variety & several days for fish to adjust to a new tank. What size is your fish & tank? Any other fish with it?
I haven't kept gold nuggets of any species (yet, on my dream list). I know they often like warm temps, ~80F. I don't know if they like wood to graze on, many plecos do. It might help to add some. You can look on planetcatfish, it helps if you know its L#. Scotcat & seriouslyfish are good too.
I'm interested in how it does does for you, good luck!