I've had close on 25...and at some times, 30 mbuna in a 55g. Water changes almost every other day though, gravel siphoning, the works. I don't have any issues with overstocking mbuna's.
Your idea of 18 should be perfectly acceptable as long as you keep the water good
As I say in pretty much every post about africans, they all have their own personalities, and sometimes the "peaceful" ones turn out to be terrors, and the ones they say are "aggressive" turn out to be big softies.
However, they do say that demasoni should be kept in bigger groups. I've seen them terrorize the hell out of the other tank residents when their numbers were low. Solved it by removing one or two other fish, and increasing the numbers in the demasoni group. Just be aware that this might happen, and be prepared to make changes if it does. I'll go ahead and hope that it doesnt
Also bear in mind whether or not you are hoping for them to breed. You might get to a stage where I was when I first started, where I wanted them to breed, but the ratio's were all off. I had to remove fish and increase numbers in the species I was hoping would breed. Careful stock planning at the start will make you happier in the long run.
Another thing to note, by giraffe, do you mean N.Venustus ? or N.Livingstonii? While I suspect they would get along to a degree with your other tank residents, you should know now that both of those species will outgrow your 55g very quickly, not to mention, N.Livingstonii eat mbuna in the wild.
Glad to see you're so enthusiastic about africans, they truly are awesome little critters !