I find that my snakes sometimes don't go for guppies or rosey reds, but I have never had one refuse a bait store minnow, particularly if I cut into it. Bait store minnows are ten times stinkier than rosey reds!
I also find that bait store minnows have thus far been a safe food for my snakes... have not had any thiaminase problems thus far... but I alternate between worms and the minnows. Pinkies are a rare treat in my house.
For small snakes, I cut the minnows into pieces, making sure that the spine and ribs are not jutting out along the edges, making a rough surface.
These are Crappie minnows, should anyone need that info. I freeze them alive (drained and in a zip lock, spread out so they are a single layer, otherwise they freeze in clumps and cannot be broken loose after freezing).
When I thaw them, I do it in scalding hot water (almost boiling) and leave them float for 2 - 3 minutes. It's my theory that the scalding water helps break down the thiaminase.