welcome phil lots of good advice on here. I recommend you research thoroughly your stocking for an aquarium this small. From memory the 620s are 620hx380dx750h and although they are 100l+ all the volume comes from the height which is of no use to cichlids. They are very territorial and in a tank that small there is no room for territories and if you stock with mbuna(dems and yellows) these are rock dwelling fish so you will have alot of empty space in your tank unless you stack the rocks pretty high. If you go for a big rock pile it will take away what limited space you already have. The dems and yellows will work if you get them as juvies but they will outgrow this tank, if you consider that you need at least 15 dems to control aggressin and if you want a colony of yellows 1m 4f thats gonna be 20 odd fish that reach around 10cm as adults.
You may get away with a 1m 4f yellow lab colony but but it will be hit or miss. This is just my opinion, not saying it def won't work but chances are high it won't the tank is to small. Have you thought about angels i think the do ok in tall narrow tanks. Like i said, I'm not an expert and this is just my opinion. I setup my first cichlid tank in a 4fter and stocked it with fish that would get to big, 6mnths later i had to upgrade to a 5fter, should have gone for a 6fter in hindsight. With cichlids you can never go to big with a tank