I added an 11" jag with a 10" Oscar. It went surprisingly well andthey have lived together for about 2 months peacefully. The jag, however had lived in a community setting prior to my ca/sa tank. My tank is a 240 and the jags previous tank was'a 125. Regardless of aggression, that's a whole lot of fish for 90. Just be prepared to split them up of need be.
As said before, a 90 is too small for a jag alone so be aware of needin an upgrade in the near future