I saw a good but not complete answer to this above. As noted above, no you dont need a DI, these have DIs built in. But note, you will see an ‘inst’ button the the Focusrites. That is the instrument button, turns that into a di. Otherwise, it is looking for line level quarter inch input, e.g. like from many synthesizers. It would work anyway, but will be a low signal and be noisier.
I also wanted to note, based on the above, I have collaborated woh people from Reaper to Ableton and vice versa. We had no problems doing so but you do need to have some care to set things up the right way. Mainly, youll want to zero the track up precisely. Simply, you just need to have the renders starting at the same time. That’s nothing to do with Reaper or Ableton, just noting that basic thing since you said youre new.