You could try any type of [b:3fb1d12fda]Synodontis[/b:3fb1d12fda] catfish. Mine munch on the algae all the time. You said you have a 75g setup, so you could put quite a few of them in the tank. (They like the buddy system anyway.) Since they are found in Lake Tanginyka, they tolerate the high pH well.
They do not clean the glass like a Plecostomus would, however. If you wanted a Pleco, I know some people keep the bristle nose variety. You should get one that is young, so if it has not been raised in a higher pH environment, it can gradually become acclimated to it. I had two Plecos (one 11", the other around 8") when I changed over to Africans from tropicals and they nearly died from even a gradual shift in the pH. I didn't get any more in my African tank because they made a huge mess. They would leave these long fecal strings all over the place. Yuck! Anyway...
I have the exact same fish as you mentioned in your tank.
[quote:3fb1d12fda]Melanochromis auratus - 5" male
Melanochromis johanni - 3.5" male
Pseudotropheus lombardoi - 4" female
Pseudotropheus socolofi albino - 4" male [/quote:3fb1d12fda]
Plus I have a male lombardoi, a female venustus, a male Red Zebra, a female L. careuleus (electric yellow), and a Debawi catfish. Check out this link to see them.
They were all purchased within two weeks of one another because I had read that if you get them when they are small (and they were 1 1/2" max), even though they are agressive, they will work out a sort of heirarchy among themselves. This seems to have worked well for me, since I don't have any real aggression problems. I would not recommend this combination for you, though, because you already have these fish and they are larger than when I first got mine. The only exception would be with the Labidochromis careuleus or Electric Yellows. They are not very aggressive and if you buy only one male with several females it might work well for you. Maybe you might try some other Labidochromis sp. or Cynotilapia sp. since everything I have heard/read about them says they are not agressive in tanks larger than 55g. It's just a suggestion!
It was not until I had all of my mbuna in the tank for two weeks that I added my three Synodontis eupterus catfishes. They don't take any crap from the mbuna, and they are very fast when getting away from them. Plus as [i:3fb1d12fda]SGypsyMermaid[/i:3fb1d12fda] would say: they are elegant looking.
Here is a list of some of the 40 or so Synodontis sp.: (Thanks to [i:3fb1d12fda]jnorris[/i:3fb1d12fda])
Just one more fish...I sware it's the last one!
Just keep swimming. Just keep swimming. Just keep swimming, swimming, swimming.