The usual advice people get here is to keep demasoni either in a big group (12+) or just 1. Anything in between they tend to fight and kill each other, in a big group the aggression is usually distributed between them enough that fish aren't singled out and killed.
In your case I'd either do 12 demasoni and 1m/3-4f Labidochromis caeruleus as the only fish in the tank, or skip the demasoni and go for something else.
Could you recommend other species you find interesting? Just to get some inspiration.
Another question, I have heard it can be a good idea to have 1-2 (I dont know the english term for them, but fish like: Ancistrus dolichopterus ) On danish: Sugemaller. But is there any concerns about having these between my fish?
A lot of people keep bristle nose plecos (american term for the regular sugemaller) with mbunas, I don't think that'd be a problem.
As for species I haven't kept that many species. I would recommend that you stick to the smaller and more peaceful species since the tank isn't all that big by mbuna standards.
I'm in the process of starting up a P. saulosi species tank myself, I like that they won't get that big and that you get two colours since the females are yellow-orange and the males are blue with black stripes.