The ministars sold by IPSF are a species of “micro” brittle starfish, so they’ll stay small forever. If your hitchhikers have been in your tank for a few months and are still “micro-sized,” I’d guess they’ll stay small too (which would mean they’re micro brittle stars as well).
Depending on the species, it may be possible for them to be babies from your larger brittle stars (but it’s rather unlikely). Most likely, your little stars that don’t have the “spikes” on the arms are just a different micro brittle star species.
As a note for anyone interested here, some people use the presence or absence of the “spikes” - called spines, but spikes is close enough for me - to differentiate between “brittle stars” (with spines) and “serpent stars” (without spines). Most of these “serpent stars” are good CUC/reef-safe, but the large, green “brittle star” (sometimes called the “serpent star”), Ophiarachna incrassata, has been known to hunt and eat sleeping fish.