Yeah it's ok when the B teams play in the playable league. I also think B teams not in a playable league cant move into one.
Screen shot show a young player I signed on a free. His stats for the B team are great and has progression arrows are on the up. My under 19's staff are better than the B team's so would I be better moving him back down?
I suppose it's the same of loaning a player to team not in a playable league?