If you're just using audio tracks and placing small clips of sound on them (eg: bass on track 1, snare on track 2, etc) then you should probably keep them seperate if you want to use this as a sort of template and change the tempo for different songs. Combining them on one track would probably get messy once you changed the tempo. However, Reaper does have that amazingly funky 'track speed' slider thing under the master volume. What an amazing idea that was!.. like having a tape speed control on a DAW...sorted
If you used a VSTi/Soft Sampler of some sort, you could create a drum beat on one track and change the tempo to your hearts desire. There are a lot of free demo soft samplers out there that should do the job for laying down a basic beat or loop. If you have a loop going, you can change the pitch of it to make it fit different tempos.
Don't really understand what you mean about 'importing into the metronome'