One of the species I've kept them with is red (orange) head tapajos. The tapajos geos tend to be fairly mild ime, though a bit more feisty when they're getting ready to spawn. So the guianacara were a bit pushy with the orange heads, but nothing serious, no nipping etc, more like competitive for spots they liked, etc, most of the time not much more than a standoff with a larger male orange head, little bit of back and forth, not much more. Smaller red heads they could push around some.
Standard thinking might be you'd find with more guianacara they'll pay more attention to each other and less to other fish, but at one point I split the guianacaras up and had a single with the breeding orange heads and behavior wasn't that different-- still pushy, still not mean. With the geos you have a somewhat similar fish in terms of habits, spawning sites they like, etc.
Same tank had wild angelfish and the guianacara didn't mess with them after finding out the angels couldn't be buffaloed. Have also had them with (adult) rotkeil severums in the tank and they didn't mess with them in the least.
Just my experience, but I could see the possibility for a particular feisty guianacara trying to boss other fish... until or unless they get tired of it and back the guianacara down, which is what I saw with my Peru angels. (guianacara I have are supposed to be sphenozona, parents from came from Rapps, but when it comes to guianacara, I wouldn't bet my life on distinguishing spenozona from similar types)
