Ok, some other things. I love the 55g as a malawi habitat. It's really a pretty good size for them as long as you resist the urge to overcrowd. The key is successful decorating of the tank. You have to have height in your decorations, it's very important because this helps alleviate aggression. When there is nothing in the top of the tank they chase each other mercilously. If you have the money, go get Texas Holey Rock, it's natural and it's probably the best alternative. If you don't have the money, get creative, I personally didn't have the money. I stacked decorations on top of each other amongst other things.
No Julies. Thankfully you know not to mix the different lake varieties of african cichlids.
I'd recommend adding a hardy and aggressive mid sized (6-7") pleco over another syno. 2 syno's should be plenty enough for a cleanup crew of the food and then the pleco will take care of your algae instead of having to scrape it off.
One thing I would say to spend your money on is a good canister filter, you can make a waterfall out of them(I know you talked about that). You shouldn't need another filter with a good canister. You can get them online for a decent price, much less than at the
lfs.
Also, your friend is correct on the harem suggestion, if you are intent on buying juvenile or adult fish. Since you have no intention of breeding, that is not necessary as long as you buy the fish at a very young age and raise them together they will be fine. My current 55 g mbuna tank has 8 fish, 4 different species, and only 3 females and is fine(and all are full grown). This is because all were raised together from about the 3/4-1" size. It's cheaper to do this and they are actually very tolerant of one another. If you try mixing them once they are large enough to determine sex they don't do as well together and you most definitely must keep them in harems. Plus if you do this you can space out your buying of decorations and also you don't need to add syno's or pleco until the cichlids are around 2"
Keep the questions coming