Command R is not workingrestart, command R while hearing the chime, select .dmg either the USB or DVD and follow instructions.
Holding the option key will only show the boot disk, I can't see the usb driveHold down the option key while booting. You will probably need an wired, official Apple keyboard to do this however.
Probably will go for an SSD but not at the moment ;-) Dont know what kind of memory i need for the mini nor how much they cost!
Are u suggesting that this can be done with disk utility?...or create a partition if you want to keep el cap.