First, I'd let the tank go at least a month on cycle and test the p.h. to avoid issues. Some crushed coral will help buffer it once things settle down with the cycle. The filtration will eventually need to be bumped up a notch, many different directions you can go depending on taste.
OK, the big issue I see is the live stock you state you are interested in. The only way you can get that many afrocan cichlids in a 55gal is the tiny shell dwellers and 20 would be a real crowd. I'm not an expert on afrocans, but know from experience they are highly aggressive and get moderate in size. They also tend to command more territory than other cichlids their size. Think of 1.5 the size territory of the moderate size central americans. If you go to Bluegrass aquatics, they can guide you in making good choices and decisions. Their fish are pricey, but worth every penny. Nevertheless, they can tell you what to stuff safely in a 55gal. As for adding all the fish at one time, absolutely. If you don't you can sit back and watch the new guys get ripped to shreds right before your eyes. Letting the tank run and cycle with a used filter and placing buffer material in just prior to the fish should keep any potential water quality issues to zip.
OK, the big issue I see is the live stock you state you are interested in. The only way you can get that many afrocan cichlids in a 55gal is the tiny shell dwellers and 20 would be a real crowd. I'm not an expert on afrocans, but know from experience they are highly aggressive and get moderate in size. They also tend to command more territory than other cichlids their size. Think of 1.5 the size territory of the moderate size central americans. If you go to Bluegrass aquatics, they can guide you in making good choices and decisions. Their fish are pricey, but worth every penny. Nevertheless, they can tell you what to stuff safely in a 55gal. As for adding all the fish at one time, absolutely. If you don't you can sit back and watch the new guys get ripped to shreds right before your eyes. Letting the tank run and cycle with a used filter and placing buffer material in just prior to the fish should keep any potential water quality issues to zip.