Hi,
I want to delete some non-sellers from microstock sites and send them to Alamy (RM). It's not that these shots are bad but mainly poorly keyworded and edited. Besides they would have fitted more with Alamy, but what did I know back then. Is it OK to delete them as RF if they never sold (or maybe once) and add them to the RM collection?

It's fine to do that if they never sold, but I'm guessing not if they sold even once.
I know you shouldn't have RF-anywhere on sale as Alamy-RM at the same time, and I'm interpolating that that would include anything that has been sold with an RF license. That's inference on my part, and that's all you'll get here (Unless someone has recently contacted them directly on the same issue. I emphasise recently as it's only recently that they codified the 'not RF-RM' rule. Best to contact them directly for an official answer.