Personally I see a few potential problems. Ordering that grab bag, you are probably going to get a problematic amount of males to females. Mbuna need a harem, or all males (which can be drab) I hope this customer realizes that mbuna are not an easy fish to keep.
Second, that filter will work fine IMO, but I would rather have two smaller filters than one large one. Or a backup of some kind. AC110 would fit the bill quite nicely.
If you only have a single canister with no other filters, that is a surefire way to ensure the canister has some sort of problem. Of course that could just be my luck...
Those pebbles, I am guessing you mean pea gravel, are gonna be poop traps. Lots of build up, and thus thorough gravel vac's will be required. Sand might be a better option. Plus Mbuna like to dig.
HTH!
Edit- just reread it, your first Mbuna setup, gotcha!
3 females to 1 male is advised. More if you have the room. A 55g is tight quarters. Very tight, but doable, if you keep the aggression down. You really cannot fathom how mean these guys can be until you have kept them and seen it first hand. Improper male to female ratio is a great way to get all the females killed, and all the sub dominant males. meaning you will eventually have 2-3 males left.
A relatively cheap option that might look a little better than terra cotta pots, would be pvc caves. Drill the pvc, coat it with silicone, and roll it in sand or pebbles. Or you can just silicone rocks to it.
Strawberry pots also work pretty well if you can find them cheap and with a decent design.
Slate can be another option. Most rocks will work honestly, as long as they are not exposed to some sort of biocide. Check some local landscaping places, they usually have a decent selection for MUCH cheaper than aquarium stores charge.