I feel like you could up the numbers by a few, especially since you do have some tank depth (front to back) to work with.
As I understand your origional post, your tank is different than a standard 75G, by being deeper and taller than 18" each way.
3 species x 5 each = 15 fish
I wouldn't up it past 18 or 20 total, but then, I tend toward the conservative when advising folks.
The rusties (I. sprengerae) and yellow labs (L. caeruleus) are pretty "relaxed" for mbuna.
Since the cyno. afra are smaller and a little spunkier, maybe they would be the ones to up the numbers on...

btw, the L. caeruleus should max out at about 5" and I believe they will be fine in your tank.
Be sure to securely stack your rocks and provide alot of caves and sight line breaks.
You may be able to accomplish some of that with your java ferns.
Since your tank is deeper than most, an idea might be to put the rock pile through the legnth of the tank down the middle,
separating the front from the back.
That would offer almost double the caves and sight breaks.
If you want more opinions and options on stocking numbers check out:
cichlid_forum.com
They are specific to cichlids and will have boat loads of experience to share.
Keep a tough skin if you post there, as some of them can be a little impatient.
I guess because they are pretty smart and expect others to know alot of biology, chemistry and what not.

But we don't and that's why we ask questions, isn't it?
I must add... do you know these fish are omnivores and prone to bloat?
You simply have to watch what you feed them and they will be absolutely fine.
If this is news to you, do some reading on malawi bloat so you can be informed and keep things under control.
Regards,
alicem