We have an iMac that is barely still compatible with El Capitan. Right now it runs Mountain Lion okay, and actually it has a smaller partition on the hard drive for another clean installation of Mountain Lion that we don't really use.
I have an El Capitan boot USB drive and an El Capitan install app, and I was thinking about using one of those to upgrade the clean install partition to El Capitan to see how it performs, but I'm hesitant and want to make sure things wouldn't be messed up.
So first of all, does this sound like a good idea? Second, would it be better to boot into the USB drive and choose to install to the clean install partition or should I put the install app on the clean install's Application's folder and run it from there?
Also would doing this risk causing the Mountain Lion restore partition to be deleted? Are there risks of things being messed up, not being able to make Mountain Lion the default boot partition, or other weird things El Capitan would do to the main partition?