Like Colin said, I have found you can.mix Malawi and Tanganyikan. Some fish stores may recommend against it, but you can do it by the following. Do a pH water reading from the separate water sources if the fish are coming from different tanks. Some species of cichlids, in fact most do require a higher pH level. Check your own water pH and add small amounts of baking soda to increase pH level. Most cichlids like a pH level of 8.3 to 8.8. Get an API (Aquarium Pharmaceutical Inc.) PH Test Kit on line or at your local fish store for about $12. Add a little baking soda each time to your water changes. The test is easy. Add your tank water to the test tube fill line (5ml). Add 5 drops of test liquid to test tube and shake it and compare it to the color card and raise your pH by adding baking soda or lower it by diluting it with clorine free tap or purified water.
Now some chilids like my electric blues from Petsmart have been bred for generations to live in the normal pH range of 7.6. Be sure and check. If you have cichlids from different pH ranges raise the lower pH slowly and lower the higher pH gradually until you get an average for both. Do this slowly over a day or two.
Now matching personalities. Study for potential future fish for full growth size and personality types. You don't want an electric blue cichlid who maxes out 2½ inches in with an aggressive bumblebee chichlid that will grow to a Big 5-6 inches. The smaller fish will end up dead or hiding all the time. Get a large group if you are going mix up species. The constant shifting of fast moving fish constantly causes bully fish to refocus their targets.
Create caves and alcoves one for each fish. Cichlids are very territorial and need their own little space.
If you are putting in several of the same subspecies of cichlid you may eventually have to separate out the males, as they mature as they will fight for the harem of females.
Anytime adding a new cichlid to a tank that has an established community, change the entire interior of the aquarium so it becomes unfamiliar and everyone has to start over evenly establishing a new territory.
All this being said chichlids are hardy and long lived. They can get to know you and be entertaining with their antics. And their vivid color patterns are very pleasing to the eye.
Keep on eye on things, especially at first. You may have to remove a bully or a fish that is constantly picked on to another tank, or donate it back or trade it in at the fish store you purchased it.
One more note, if you are using your tap water and the pH is naturally, exceedingly high; then dilute it with purified, reverse osmosis water or add calcium or powdered sea shells. Have fun and I will be happy to help you as much as you want. Oh and the API PH Test Kit has MANY tests in it. Since you are starting out you might consider API Master Kit. Or buy the separate kits for Ammonia, Nitrate, AND Nitrite. These are toxin cycles in your tank that need to be neutralized with water changes and Prime or Safe. Be sure to do frequent water changes and start with a few fish to allow your natural beneficial bacteria to culture and cycle. It takes about three weeks unless you can borrow some gravel and or water from an established tank. See if you can rub your filter pad in the gravel of an established healthy tank at your fish store and keep it warm and moist in a plastic bag until you get home. These kinds of jump starts can cut the cycle time of your tank in half. For a new tank I precycled new gravel and filter by soaking it in my established tanks for a week. Then I added ⅓-½ cycled water from my tanks and the new tank cycled in a week instead of 3-4 weeks. Enjoy and you will find all of this very interesting and satisfying in use.