Siamese Dolls

Went into my aquarium store today (the "lesser" of two local ones) and
they had in stock a number of Siamese Dolls. Some were albinos with red
eyes and yellow-gold coloring. I got two fantail calico that have the
look of Ryukin. The thing is, these being for my 15H aquarium, I got
them because I was told they don't grow any bigger than 1.5". Reckoning
that's probably an under-estimate, even if they get to be twice that, I
should be all right, right? They look very nice, and have a deep orange
inside the head area, under the skin. At the shop, I saw some of these
fantail ones nipping at the tails of the plainer albino ones, which I
thought was odd. I would've thought it would be the opposite way.