Depends what you want to do.
does mixing lakes bother you?
does mixing regions bother you?
how much you want to spend?
300 is a nice size tank, and things that would not work in a 55 or a 125 work much better in a 300 behavior wise.
If sticking with Tanganyika.
6-8 frontosa.
4-6 Benthochromis tricotti.
5-6 kilesa, for the bottom, males get 6 inches, the females may get picked off by adult frontosa.
6-8 synodontis multipunctatus.
then for color either some leluipi or Astatotilapia burtoni ( a victorian like fish that is found around the swamps around Tanganyika.
If willing to mix lakes.
6-8 frontosa.
5-8 C. moori, or better yet, Placidochromis phenochilus for the blues.
6-8 electric yellows for the yellows.
4-6 Albino Eureka Reds for the red/orange.
3 snow white socoloffi for white.
5-6 Synodontis multis.
you may get some nipping by the yellows and socoloffi against the frontosa, maybe not, personality varies alot.
If willing to mix regions.
6-8 frontosa.
5-7 red head tapajo's, or something easier to find such as Gold Severum, i know some for sale cheap large.
4-5 Pin stripe Damba from Madagascar.
some Synodontis.
a nice big sunshine pleco, a nice big L 024 scarlet pleco.