I might want to add hardware compression to individual tracks after I have recorded them...I'm guessing this can only be done with a mixer?
Erm... no. You can do that using your interface now, you just need to work out how to route individual tracks to your compressor and then back in again, and by the looks of things, your interface (and for that matter Reaper) makes it pretty easy - at least once you have got over the learning curve...
Its pretty much as Lowe said:
1. Have a track you wish to compress using an external compressor.
2. Insert ReInsert into the insert effect of the track you wish to compress in Reaper.
3. Set the correct in and out channels corresponding to your hardware configuration.
4. Thats it. Listen. Compress.
5. When you are ready, record the output of the track onto a new track.
6. Profit.
Will a hardware mixer
really help you with this? Maybe, maybe not. You'll still have the learning curve. It really depends on how you prefer to work (and your budget). But lets face it a hardware mixer is a darned pretty thing
To be honest you must have a pretty special hardware compressor to even be worrying about this. For two reasons, one - its a pretty basic function and really nothing a modest software compressor cannot do (unless yours is tube based and has a certain tone/warmth maybe, and even then there's options in software form) and two - you already have a compressor built into your Roland Interface (although to be fair, its probably rubbish).
I live in the software world and really only consider external hardware processing for Reverb, and Mastering (mostly). This is because any decent software reverb or mastering algorithm takes a bunch of CPU cycles and stymes system performance on large projects and frankly, its a massive pain in the backside to set up and more importantly, reconfigure when you want to come back to the track 6 month later. That's just me though.