I used to have a Rossi .90 and I never could get it to run very reliably. From what I have been told in the past, they run a little cooler with less oil than normal DF fuel and that some people use 4 stroke fuel in them.
I was new at df at the time and it was more of a headache than it was fun so I ended up selling it. Come to find out a couple years later that the Rossi's shortcoming is in the carburetor and if you put the OS carb on it the engine is a real powerhouse developing more power than the OS .91. Kinda wish I had a kept it now!

I'm only going by what I experienced and what I have been told in the past but I'm sure there are some Rossi experts out there that can shed some light on this topic.
Bythe way, what engines are in the two planes that you bought?

Good luck!
Myles