No, you should not under any circumstances be adding the incorrect fluid to a transmission. If the transmission in question calls for ATF use ATF. Likewise, if it calls for CVT fluid use CVT fluid. The couple dollars you might save by using some left over fluid will quickly be erased by transmission failure at worst, or needing a flush at best.
After seeing this video (not mine) of the squeal and dashboard, a Subaru specialist diagnosed the problem as a worn bearing in the Torque Converter Clutch (TCC). This is a known issue with this generation of Outbacks and was one reason for the recall on the CVT transmissions a few years ago.
They said I could continue to drive the car without damaging it....
So, simple analysis is:
max belt speed <= engine speed * gear circumference
Throwing in the numbers and solving for the diameter at the maximum point:
Edit based on a unit error...
D = 35 / ((3700/60) * 3.14159)
which gives 0.18 metres ie 18cm.
That should enable you to work out what you want to use for a final drive ratio..