If your wanting to add more fish your definitely going to have to upgrade your filtration mate ..Stocking wise you could add to the groups you already have, 2 to 3 males and 6+ females of each species and then go for 1 more relatively small species like pseudotropheus saulosi/pseudotropheus minutus lupingu/Iodotropheus sprengerae quite a few to list really. What are the dimensions of your set-up? 180L is a little small so your a little limited to what species you could keep. You got any idea's on what other species you like the look of but not sure if their compatible together? Instead of me just reeling of loads of species (could be here all day lol).