This is personal opinion, but there isn't a chance I would take over the established BB in this situation. You are literally getting a grab bag of potential issues doing so. I'd sterilize the tank and all of the filtration and media and get you a clean cycle going. I know it is tough to wait the 5ish weeks for a new cycle to begin, but it might save you loads of heartache in the future from dealing with someone else's problem.
Years ago I had a similarly stocked 125 gallon, here are some things you might want to consider.
My stock was:
1x Oscar
1x JD
1x GT
5x Silver Dollars
3x Raphael Cats
If you know for sure that your main fish will be an Oscar, then you should get the silver dollars. Oscars are notoriously messy eaters, they are like bull dogs drinking water, 50% or less of what they put in their mouth actually stays in their mouth. Silver Dollars will gladly clean up all the little bits the Oscar chews out his gills and it will happen in a matter of seconds. I had standard silver dollars, they grew incredibly fast and were awesome to have in the tank.
Secondly, on the Raphael cats. If you have hidey holes in your tank, you will NEVER see these fish. Seriously, you will put them in at 3" long and will not see them again until the next time you are moving stuff or cleaning substrate and suddenly they are 6" and incredibly fat. They do a really good job of cleaning up the bits that the silver dollars missed.
I have Severums in my 240 now and love them, but they are like any other cichlid though and can be hit or miss. I have two that are awesome fish and don't bother anything, and really only care if they are being fed or not, but I have one that is a total jerk on some days and an angel the next.
Ultimately, good luck in your decision, however, I would STRONGLY recommend you start over with clean materials.