I have made over 60 RPC's and Static Convertors , but have never run or worked on a 10 EE.
Using a 10 HP Idler for a 7 1/2 Hp motor should do fine but let me explain
One of the considerations that most folks ignore is whether the lathe has a clutch or not (important)
If your lathe has a clutch , then it will always start without a load ( assuming the operator knows good practice) and
not have any issues with a 10 HP for the idler . If the Lathe does not have a clutch then starting loads increase
and reflect the work at hand, like trying to start in high speed with a 20 pound chunk of steel in the chuck.
Now, I did have a problem one time with a WEG motor, made in Brazil ,
that was a tough cookie and was in a Nardini lathe and was 2 speed 7 1/2 HP motor.
It also had a auto-brake and no clutch. I found the difference in load amps between low and high speeds as incredible.
I think a 25 HP idler is way overkill , and stay away from WEG motors
Don't forget, that you can add some running Caps on the slave motor in addition to the RPC to get balance
Rich