After a rough read through the update notes, I think it reads that there will be some new rainbow FKs and FMs after the mentioned maintenance that will not be available in any gacha lineups, but will be available in the rainbow medal exchange corner.

Also, it seems the amount of units there will reset to max from time to time.

At least, that's what I believe it is saying at the moment. I'd need to take my time with it to know for sure.