Suitable Malawi Mbuna Tank Mates

This is my first question on these forums, so bear with me.

I just set up a new 55g home for my Lake Malawi Mbuna Cichlids, and I'd like to explore the possibility of adding a few non-Malawi tank mates.

Currently, the tank houses one each of the following cichlids:
Acei, Kenyi, Auratus, Maingano, and Red Zebra. It is also home to a small clown Pleco.

Tank is standard 55g, 48 1/2"L x 13 1/2"W x 21"D
pH is kept at a constant 8.2, KH stays ~10, and GH ~5, and rift lake salt is added at 3/4 tsp per 10 gallons at every weekly 25-50% water change. Ammonia & NitrIte are always non-existent, and water is changed immediately if NitrAte is over 40. I test at least weekly with API tests, for high pH, Ammonia, NitrIte, NitrAte, KH, and GH. It is currently decorated with lava rocks with holes, black sand & gravel mixed substrate, and fake plants. I plan on replacing the lava rock with better rocks & cave structures in the near future.

The pleco doesn't seem to have a problem with any of it at all, even though the parameters are at the high end of its tolerable conditions. And the cichlids don't even seem to realize he's there. If they do, they sure don't care.

So far, the suitable tank mates I've read about on the web consist either of other Lake Malawi Mbunas, or fish that will get along with a cichlid living on their turf, but can't tolerate living on the cichlids home turf due to water chemistry. It's not a strict biotope setup, but it is first and foremost a Malawi tank so I'm not willing to make any major chemistry changes. It would just be nice to get a little more variety in the tank while keeping everyone happy and healthy. If it comes down to it, I'm perfectly content with just keeping it stocked with Malawis.

My criteria for choosing tank mates are:
1. Must be tolerated by the current inhabitants.
2. Must tolerate water conditions of pH 8.2, KH ~10, GH ~5+, lightly salted, temp 76-80.
3. Must tolerate a similar diet (herb/omnivore), to reduce risk of having too much high protein food in the tank available to the Malawis, which could potentially lead to the dreaded bloat.
4. Must have an adult size of between 4-8"
5. Other cichlids are fine as well, as long as they meet the same criteria and are relatively commonly available in LFS.
6. Personality goes a long way. But failing that, pretty colors are always nice too.

I hope I've provided enough info. As I mentioned, I'm new at this, though I have a ton of experience pouring over posts like these found through Google. Unfortunately, I haven't been finding much that's of any use this time.

If anybody has any ideas, experiences, or warnings about tank mates, I'd love to hear them!

I appreciate your input. To answer your questions...

Right now I've got a Penguin 350 Bio-Wheel filter on the back, which I transferred directly from their old tank without cleaning, full tank to full tank, matching water perameters, media and all, to aid in cycling the new tank with a minimum of beneficial bacteria loss (I did the same with the decor as well). When I purchased this tank setup, I also received a Penguin 200 Bio-Wheel filter which if still in good working order will be used alongside the 350 as soon as I get it cleaned up and purchase a new wheel for it (the wheel on it has long since dried out). Manufacturers ratings on the filters are 75g for the 350, and 50g for the 200. I figure 125g of filtration for a 55g tank should be sufficient with weekly water changes, so my stocking concerns have more to do with the happiness of the fish living together at adult sizes more than their effect on water quality.

As for the salt, I use it to help buffer the water and add trace elements like phosphorus and calcium. I began using it upon the recommendation of a trusted employee at a LFS. Not a big box pet store, but an actual LFS with a knowledgeable staff. He has kept cichlids for years in multiple tanks, and is always full of advice he has from personal experience. Yet, while I tend to trust him for advice, I always do my research. The trouble is that on the topic of mbuna salt, the opinions vary greatly. I find sites that say Lake Malawi doesn't have any salt in it, and others that say it's one of the saltiest lakes in Africa, if not the world. Rarely does anyone make any distinction between mbuna and the rest. Usually a majority of the people on each site tend to share the same opinion as others on that site, while another site will have people that all share a contrary opinion. In the end, I figured that it shouldn't hurt anything, and would just be a matter of potentially spending more money than I would otherwise have to.

I know what you mean about the Kenyi and the Auratus. I haven't seen too much aggression from them yet, aside from them chasing each other around a bit, but everything I've read points to them being problems down the line. They were my first 2 cichlid purchases, prior to when I began researching the fish I buy or am interested in buying. Knowing then what I know now, I would not have purchased them. In fact I have recently been considering re-homing them.

Funny you should mention yellow labs and rustys, as I was actually just considering them when I was pondering tank mates at the LFS the other day. So far, between stock available at a trusted LFS in town and web research, I'm considering labidochromis caeruleus, labeotropheus fuellerborni maleri, labeotropheus trewavasae, and iodotropheus sprengerae. I still need to check with the other good LFS in town to see what kind of stock they currently have. This second shop can also get ahold of less common cichlids, usually within a couple of weeks, which does increase my options as I'm in no real hurry. Better to stock right than to stock fast!

I've actually been considering petricola, stumbled upon them last night while going through various forums. Glad to hear they are working out for you. That's a vote in their favor. I'm also considering Synodontis multipunctatus, but for 55g he may be a bit large for a well stocked tank.

I hadn't considered tropheus, but upon doing a minimal bit of quick research since reading your reply, they do sound as if they would work out well with my mbunas. At the least, it sounds like they enjoy the same feeding & water preferences. I'll have to look into their aggression and personality to know for sure.

As for my clown pleco, I'm sure he'd be fine in my 46g Rainbow/community tank (soon to be upgraded to a new 90g setup), so it would be no big deal if he needs to move out.

Thanks for all the replies everyone, keep them coming! I'm beginning to feel like I've got a decent variety of options brewing!