Sounds like a great idea!! I think Malawian Community tanks are the best. You are asking great questions, and I am impressed that you are thinking ahead, and getting a plan of attack so to speak!
We could argue filtration until we are all blue in the face! Get what you want, what ever you feel is easier to work with. This most important thing is to have plenty of it. Also remember just because you have plenty of filtration, that you will still need to do water changes. I have about every way you can do it, set up, I use sponges primarily, but I have larger tanks in the house, and I have UG with power heads, Outside Power Filters, cannisters, etc.
I don't like to glue to many rocks together in my tanks, I move the fish around too often, so I like to be able to take it all apart. Then I put it all back together, I know sounds like a pain, but I never get the same pile of rocks twice!! You just have to be careful and use larger rocks on the bottom and smaller rocks on the top This again is just a personal opinion. I have seen tanks with rocks hooked to the back of tanks before and they were cool, I have seen pottery also that was neat. But the latest is the rock that comes from Sweeden I believe, it is a rubber imprint of a side of a mountain I guess. And you put that in you tank for your background. Expensive but cool! It is featured in, "Enjoying Cichlids" Ad Konings, Editor. I saw it at the ACA convention this summer and unfortunately I must have been in Girls room when they auctioned it off.
If you like plants go for it. But beware that Mbuna are vegeterians and they will probably eat them. If you don't mind, then it won't be a problem. You can always use my personal favorites... plastic.
How many fish can you put in there?? Well that is a tough one. Depends on what species you want to work with. How many rocks you end up using. By the time you get gravel and rocks in a 65 you probably only have 45 gallons of actual water. I would say go with 18-24, you are going to loose a few. That would give you 4 different species, 6 of each.
I vote no on a pleco, everyone knows pleco's suck!! Yes on a Synodontis!! Put some class in your tank!