Every fish is an individual. Keep that in mind. I've heard many people say that kribensis can be aggressive with other fish. I've only had one, so not much experience, but she never bothered anyone in a community tank. Of course, the tank was a 55, and more room will sometimes mean less aggression.
Angelfish are cichlids, like the kribensis. The Angel could have been somehow stressing the krib out, making her feel vulnerable, even if it wasn't attacking her. So in turn, the krib could have been taking out her aggression on the dwarf gourami. Also, it's hard to say, but dwarf gouramis are not the most peaceful of fish, and maybe the krib felt she should assert herself before the gourami did. Also the red-tailed shark could have also made the krib feel defensive. I know an awful lot of people on message boards have not had luck with kribs getting along with other fish, so it really is not uncommon.
Cockatoo apistos can be little pistols! I had one, and she was smaller than any fish in the tank, yet she still harassed and bullied them constantly. Use caution with apistos! They are firey! I definitely wouldn't add both a GBR and an apisto. I had trouble there too. A GBR would likely get along with what you have in the tank, as they stay toward the bottom a lot, and your others are top/middle dwellers, (except for the pleco) but there is always the chance it won't. Also, keep in mind GBRs are known to be delicate, and people report many unexplained deaths, myself included. So they can cause heartache in that way. And yes, a GBR prefers temps of about 82, whereas a majority of tropical fish like it at about 78, so that is a four degree temperature difference. You have to keep that in mind, as I'm sure you don't want any of your fish uncomfortable in water that is too cold or warm for them.
GBRs do fine by themselves. Lots of people have them as singles, as well as Bolivian Rams. A pair would most likely be very aggressive when breeding, and that would lead to aggressive toward other fish. So you have to keep ALL the factors in mind: Disposition of the fish, temperature, top/middle/bottom, dwelling, etc. It all has to balance out or there will be trouble and/or deaths.