The Midwest Industries Enhanced AR15 Drop-In Trigger might work with a Rimfire AR15. However, since Midwest Industries does not state whether or not the Midwest Industries Enhanced AR15 Drop-In Trigger is compatible with Rimfire AR's I recommend contacting Midwest Industries to confirm compatibility.
Yes, the Midwest Industries Enhanced AR15 Drop-In Trigger is designed to fit and function in most .308 and .223 AR platform rifles and pistols.
Yes, the Midwest Industries Enhanced AR15 Drop-In Trigger will work in a Radical Firearms RF-15 chambered in 7.62x39. The Midwest Industries Enhanced AR15 Drop-In Trigger works in Mil-Spec AR15 lower receivers so it should work just fine in that rifle.
Yes you can.