I have 2 sevs with 1 oscar. Oscar tries to pick on them... then he goes too far and WHAM! severum fights back. No torn fins or anything like that at all. Jags I think would be different since they are more aggressive overall. But it is still up to what individuals you have, every fish seems to be different.
It's been my experience that if an RD has something closer to it's size in the tank it generally will ignore most of it's smaller tank mates. I have an RD in my 125 with my Sevs, a Uaru and Texas cichlid. I've had this set up for over 4 years and the RD chases the Texas from time to time and ignores the sevs and Uaru. My RD is closing in on the 18" mark while my Tex is a good 13". Tex has a nice hiding spot when the RD gets too rowdy for him. But I wouldn't try putting Sevs in with my RD if there wasn't something else for him to chase around once in a while.
The Severums in my avatar are in a 150 with a 8" jag,8" dempsey,6"lyonsi,10"xingu pike and a few others. The JD had a few runs at them when he was first introduced to the tank but that only lasted a day or so
Now nobody messes with them