I have a 90g that (before I reorganized) had this setup with an assortment of upperjaw poly's, the ornate I had (ornament mishap) did not grow very quick so it probably could have been comfy in a 90 for a year or maybe 2, but(!) not for life, these guys get HUGE.
I also had oscars in there, but they were juveniles. In this tank I now have the same oscars and a shoal (x7) of normal silver dollars, a marbled knife (~6")(P. Afer) and a sydontis catfish(~3.5"). What I have found is these silvers school together very tightlyand are always there for scraps dropped by my oscars. I also found that they are not the hardiest fish-the redhooks might differ. The biggest difference between the redhooks and normal silver dollars is that the redhooks get alot bigger, mine after their initial growth spurt are 4" (the biggest male) and the smallest of the first 4 i got is ~3". The redhooks are supposed to easilly get to 6", this may cramp the 90g.
For just a 90 I would say start with 5 redhooks and an upper jaw species of poly (delhezi, senegal, palmas, etc) or change it up to an endiler (sp?) lower jaw as they are supposed to not get much bigger than 12-14" in a home aquarium. -I'll let you know when mine grows up!
If you want a stunner tank, 10 redhooks and x2 ornates in a 125! And maybe a scavenging sydontis cat...