If you go really light on your other fish, your bioload could be okay in a 40. Say, if you had a snowflake in a 40, I wouldn't put anything else in with it other than a pair of clowns and an electric damsel or something similar. What I would certainly plan to do is run a 120-ish capable skimmer in a 29 sump carrying about half its capacity (about 15 extra gallons of water). It would be an extra step but a vinegar drip will increase its efficiency if you want corals.