Instead of linking them, why not just pull some rock out of the established system and put them in the new one?
In terms of allelopathy, I think it's always present, just that the negative effects are not always evident. For example, I believe that my toadstools were affecting my gig. When my toadstool got very big and dropped babies, my gig would stretch out more. I removed the last of the toadstools just the other day, and the gig looks a lot more like a normal gig -- it's no longer stretching its column and has deep folds of a typical gig. I admit that it could be a coincidence, but nothing else changed in my tank. I'm curious to see if other gig owners have toadstools or other softies in their tanks.
In terms of gigs and hardiness, IME they are VERY hardy once they are established. If your LFS had them for 2 months already, they are already acclimated to captive life and should be fine. I suggest taking some updated photos and posting them so we can assess their overall condition.