I have oscars, 2 red devils and dempseys together for quite awhile and have had absolutely no problem "YET"!!!
My devils are very tame and friendly with my other fish.
He swims around like there is no one else in the tank.
This is just my experience and probably just my luck so far. It also may be that I basically brought everyone into the tank when they were just squirts.

u seem to have gotten lucky. devils are usually extremely territorial and and from what ive seen rarely swim in around. (unless you have no rocks, plants or caves which i would never recommend.) it could have been because u introduced them to each other early on. but i did that and didnt have quite the luck youve had. how big are the fish?

yep think its luck but then again everyones afraid of me, lol so they better behave. I have quite a bit of wood, rocks and some artificial plants incorporated with hiding places. Sssh! but I just got me a green terror too and so far so good.

Oh! the red devil is about 4". My biggest oscar is about 7", depsey about 6" and my terror about 3".

Thanks to all who responded, just to update as it has been a few months. The 2 oscars 6",2 balas 5",devil 4",dempsey 4",convict 3" and pleco 8" are in the large tank and while the devil does seem to chase the convict and the dempsey, the dempsey quite aggressivly there are no nipped fins and when the dempsey is in his cave he will defend and move out the devil. The devil shows no aggression towards the oscars but the oscars do chase the devil every so often. The bala sharks I have always had with oscars and they are just to quick to be caught by any fish in the tank and they seem to be completely ignored by all.
Balas make a great cleanup crew and they are to large for the oscars to see as food so they are a non issue in this tank. Again, thanks for the help.

i'd love a dempsey but i haven't got the room

Hopefully everything stays a-ok in your tank welby. Sounds like a great mix.