My friends next door have had a tank for a while and it now has two oscars (one about an inch larger than the other), two plecos (one about an inch longer than the other), and one african cichlid (unsure of species). I've been over there alot, especially before I got my own tank to maintain. I've noticed that depending on the lighting, the fish pecking order changes...here's what I found- maybe you'll find it as interesting as I do.

In Bright aquarium light:
1. Larger Pleco
2. Larger Oscar
3. Smaller Pleco
4. African Cichlid
5. Smaller Oscar

In dimmed light (room lights with no aquarium light):
1. Larger Oscar
2. Smaller Oscar
3. Larger Pleco
4. Smaller Pleco
5. African Cichlid

In very little to no light:
1. Larger Oscar
2. African Cichlid
3. Smaller Oscar
4. Larger Pleco
5. Smaller Pleco

I find it strange how much the heirarchy for the Afr. Cichlid changes in particular. And find it strange that the larger Oscar happens to get bullied around by a Pleco that's smaller than it even though the Oscar was the king of the tank before the introduction of the pleco.

I think a lot of it depends on the actual fish. You'll find that some fish and more dominant than others, even if they are the same species. I have got two blue acara one of which is a real little bully towards my Eartheaters, whilst the other one is very placid indeed

I was talking to another fish enthusiest a few days ago and he said he had a 6" or so African Cichlid and introduced a about 12" Red-Bellied Pacu to the tank- and the little Cichlid would actually chase around and scare the big Pacu. The Pacu never once tried to challenge the pecking order.

I've seen a 12'' oreochromis sp. (an african cichlid) attack a pacu that was around 36'' long and much larger. The little guy really took charge in the tank(it was a 960g at an lfs) and bossed around other fish too including a marble cat, a few oscars, an a 20'' arowana. Pretty suprising to see. I have some of it on video, might upload it soon.