The angels and Discus combo can be pulled off...you just have to be carfeul because most angels will pick on Discus if given the chance, and then theres the whole Angel carry a certain bacteris that the Discus immune system cant handle...the Only exception to this are the ALtum Angels( pictured above) as they do not carry this bacteria. They are also not as "aggressive" as other Scalare...aggresive being a relative term when you're on a monster fish forum...lol. Anyways, I as well as Matt and many others have combined ANgels and DIscus with no problem....If you choose to do so, you do at your own risk and need to find the right balance for it to work...
Also - DIscus and Angels do in fact inhabit many of the same rivers and tributaries as P. Bass, RTC, TSN, WOlf Fish, Arapaima, Rays, etc...most of the monster fish though are out in open water, or the river floor whereas angels and discus prefer to stay in cover, by the shoreline, bushes, driftwood, etc.. where they have cover and protection.