I like the slate look looks like the makings of a good tank, if it were me I would be tempted to stack the slate higher at one end and maybe lower in the middle to break that straight line through the middle, but thats just my obsession with the photographers rule of thirds maybe. I have a 450 litre with an fx5 and find it to be a really nice, quiet and easy to maintain piece of kit, not sure if you may want some extra filtration once you get it stocked though, I also have an internal U4 and a 5500 newave powerhead to eliminate dead spots.
As for stocking think you have loads of choices here's my current stocking.
4 Labidochromis perlmutt - one breeding pair.
5 Labidochromis caeruleus - one breeding pair
3 Hongis Red Island - getting it on but not seen any holding yet.
1 Pseudotropheus Crabro
1 Pseudotropheus demasoni
3 Pseudotropheus socolofi
2 adult Cynotilapia afra "red top" (breeding pair) and approx 20 juves, babes not so small now and due to go to LFS this week.
6 Syno multis
1 Nimbochromis venustus
1 Cyrtocara moori
1 Nimbochromis livingstonii
No aggression, but know some people have major issues mixing Haps and Mbuna not currently any probs in my tank though, single specimens of crabro and demasoni due to their reported high aggression levels in small groups, The Labs seem more than happy. Originally had 3 Afra but turned out to be 2 males and a female and the 2nd male was seriously bullied.
Hopefully this may help you a bit or consider some fish you may not have thought about.
Good Luck.