That's a tough question. I usually work on a trial and error basis, but I make educated trials if you know what I mean. I will not try keeping a 2" oscar with a 18" gar, and I will not try keeping a super active and aggressive fish with a shy and slow fish that can't compete for food.
As for adding them all at the same time, that usually is ideal when they are small enough not to kill(overwhelm) your biological filtration, but it doesn't always work. Some fish grow much faster than others and if you get them all at the same time, one might grow so fast that withing months it will be big enough to eat the slower growing fish.
And finally, about cichlids...
It all depends on ...many things. Are you talking about GG tanks mates only? Are the gars out of the questions now? If so, then most cichlids that are not overly aggressive could work. And parrots would be a good choice. Other good choices include angels, severums, convicts(aggressive, but small), jack dempseys(depending on individual behavior), and firemouths, just to name a few. Of course, if you wanted a gar too, then a lot of these fish would be out of the question as they would become meals to a full grown gar.