I wouldn't add any new species until you fill out your current stocking, once the ones you have reach sexual maturity you're going to have some problems. Mbuna are harem breeders meaning a single male will breed with as many females as possible, because of this if you only have 1 male and 1 female, the male will constantly harrass her to breed and possbily kill her in the process. If you have only 2 males of a single species they'll fight constantly for territory.
My suggestions:
2 Kenyi - (Metriaclima lombardoi) A very aggressive species, if you decide to keep them there should only be 1 male per tank and at least 4 females.
3 electric yellow - (Labidochromis caeruleus) One of the few species where multiple males are tolerated, that said, they do best in groups of 5 or more.
2 red zebra - (Metriaclima estherae) A mildly aggressive species, should be kept in ratios of 1m/3f
2 bumblebee - (Pseudotropheus crabro) A highly aggressive species, if kept in groups should be in a 6ft species tank with only 1 male. However, a single specimen is a good addition to a semi-aggressive mbuna community.
4 platy - They won't last long once the cichlids reach sexual maturity. I'd remove them, it's not really fair to leave them in knowing they'll be killed.
1 rubbernose pleco - You'll likely never see it, they are very shy and tend to only come out at night. If the mbuna can catch it I wouldn't be surprised if they kill it.
So if it were up to me stocking would be:
1m/4f M.lombardoi
5 L.caeruleus
1m/3f M.estherae
1 P.crabro
and 1m/3f of another species to round out your stocking.