I lean on the conservative side, so the pair I have now are in a 150g tank and have no real pod competitors.
I don't keep any wrasses at all.
I agree w/ the 75g per one as a general guideline, w/ plenty of LR and a fuge.
I personally just prefer to give them the best conditions, and there are so many other fish to choose from, prefer my mandy pair fat and happy and a peaceful tank.
I think OP is pushing it for keeping one, no way would I try two, unless that 40g was tied into a pretty big fuge teeming w/ pods.
Just my opinion based on my experience in keeping mandarins.