First off if the Covict is male the Dovii wont eat it as long as you never feed it live food. With that said a 75 gallon is too small for Umbee and Dovii. Keep the Convict and add similar sized cichlids such as Firemouth, Salvini, or Jack Dempsey that might work.
If your convinced you want the Dovii and Umbee you will need atleast a 240 gallon tank and even then may not work. So if it were me I would decide on either Dovii or Umbee and a 240 gallon tank. Then you could do dithers or target fish such as Tinfoil Barbs or Silver Dollars.