90x40x45 is a bit small for Yellows and Dems.
But you may get around it by raise them from fry together. And keep decent number in each group especially for dems at least 12. Heaps hiding places, rocks, caves ect...
I cannot see anything else are better then yellow and dems for your setup at moment consider your tanks size and your requirement for colors.
May be single species of Kribs or Neolamprologus brichardi also an option.
Good luck :thup: