black belts are very aggressive, also they arent african lol.
i was gonna buy one but the store owner talked me out of it by saying they more or less should be kept alone at adult size 12" or more. pleco's can live with anything, including pirahna, just when they try and asert their dominance the pirahna's will gang up and kill him, but just goes to prove, your plec will be fine,
elephant nose is gonna live a scared hiding life, or just generally a short one followed by death.
severums should be fine as they are peacefull but as i have found they wont take any stick either. convict may be able to stand up but there will be a point when your fish will be maxed out in size, and you will have a adult 4" convict going against a 12" maybe 15" black belt, the odds surely wouldnt be good.
with anyluck the cichlids should fight among eachover as cichlids do, but generaly no harm will come from it. i sugest you move elephant nose, maybe to another tank ? to a friends? or exchange him for fish store credit.
regards, kubora666