Hmmm I would take what the seller said with a pinch of salt. They CAN grow slowly but mine certainly didn't! The two youngsters I got have grown very fast over the past month or two. Suprisingly so in fact. It may be that having the adult, rather gravid, petricola in there gave them a spur on and having a great deal of space to play and grow up in, alongside a very varied diet. I have noticed with some of my other syno species that their habitat plays an enormous role in how well and fast they grow. I had 4 Eupterus in with my gouramies at one point but moved 3 out after BB causing all sorts of trouble with them. I couldn't catch the fourth for some months. The three that were moved went in with my adult group that were at the time in my 55g. They grew pretty rapidly and now aren't far off the size of the adults despite only being around a year old. The fourth however, once I caught him, prooved to be having a bit of a rough time and was noticeably smaller. He was moved into my 44g where he joined some other synos although no other Eupterus. He began to grow and be much healthier in appearance over the following months before the move into the 100g where he now lives alongside all the Eupterus and my single Clarias and the Multi's. He is doing well but is still smaller then those who he was originally the same size as.
Hugs,
P.