Hmm,m pretty sure my other bought creatures kept leveling above that soul threshold. I bought them at around lev 4-7 and they were lev 22 if I recall correctly. The same level as my "hero". Also no new familiars were added to the shop, I had the same ones during the entire play, and I bought four of them, waiting for any new better to turn up but it never did. There were a few 4ish of them lower ones there, but since I couldn't add more than 4, I saw no need in replacing any of them.