Bait shop minnows can have problems with parasites, but if you get the silver ones and avoid rosy reds, thiaminase isn't an issue. My adult ate nothing but silver bait shop minnows for over 10 years, and is still quite healthy. If you can freeze those minnows though to kill parasites, I really think it'd be safe as far as thiaminase goes.
Of course there is always a risk though, so idk...