Some people will tell you african cichlids are high maintenance, but I tend to disagree. So here's my opinion based on keeping them for close to 8 months now.
#1) The Emperor 400 will be MORE than enough filtration for what you're talking about.
#2) The african varieties do sometimes dig, but this can be avoided. They have a tendancy to dig under ornaments. You can remove this possibility by having open hollow ornaments they swim in and hide in. They'll prefer this anyways. In addition, they will leave most plants alone.
#3) Kinda continuing the above point. African cichlids come from a kinda rocky environment. Most people are going to tell you they need caves and such to swim in and out of. I agree. I don't have any per say myself, but instead mine have two large sunken ship halves that serve the same function. These are again open bottom and completely hollow so no one can get stuck.
#4) Size... It is my experience that you can expect 4-6 inches out of these fish, depending on the species. My largest is about 5 inches, and he probably isn't a year old. I expect at least one more inch out of him, but he's slowed the past two months in growth.
#5) Tank size and number of fish. For a 55 gallon, I would cap myself at about 8-10 fish. If they all grew to 6 inches, they'd break the 'inch rule' but you have enough filtration to handle them, and I honestly believe they'd have a decent amount of space. You'll also want to consider an algea eater that grows somewhat large, so that they'll leave it alone and not eat it.
#6) I like numerating things. These guys come in a TON of patterns and colors. They're very active swimmers, and very active eaters. They love bubble wands. Some people might dispute the following but I believe them to prefer 78-80 degree water.
#7) Don't worry too much about their initial size at purchase, unless you already have some full grown ones. These guys do have a fairly aggressive pecking order, and they will fight somewhat with newer fish. My smallest is 2 inches max, but despite being picked on is still healthy and doing well. Expect to see them fight every so often and chase each other around.
So summarized. They're cool. I love my cichlid tank. They'll mostly leave your gravel alone unless you provide them something they WANT to get under. Plants should go unhindered as long as they root well. Emperor 400 can handle a 125 tank depending on fish stock, so you're well off.
hth again
