Those two species mixed creates one of the nicest tanks I have seen and infact is a very popular combo. Their colors contrast each other nicely. You often see them for sale together as package deal for that reason. As far as numbers with your tank I couldn't give you as educated an answer as others here so wait for other responses. I keep both but in a 135gal. tank. I statred with 8 Demasoni and have yet to had a death but I am a rarity and have atleast 16 in there now. The original 8 are 2-3 years old and aggraession is getting bad so I need to add more. I alo have a ton of rocks in there.
Hopfully you get good advice on numbers for a tank your size and that you try them. You won't be disappointed.
You need to keep demasoni in a group of at least twelve, it is said. Start with 20 juveniles, and fish out hyper dominant and subdominant males untill they settle.
Labs work well in a group of one male to four females. With the large group of demasoni, I don't think you should do more than five labs. Even then your bioload will be high, so expect frequent (or large) water changes.
Try reading around on the forum, there are many people who want to know exactly this.