I work in surround with Reaper all the time.
First, equipment-wise, it looks like the Octa-capture has enough outputs for surround. There are 8 outputs, so you could go as high as 7.1 Surround. I'd recommend a third KRK 10-3 for your center channel and probably some Rockit 5's for your surround channels and a sub.
In an ideal situation, you'd also have a surround monitor controller if you're changing your monitoring levels a lot. Does Roland have a software controller/mixer app for the Octa-capture? You could get by without a monitor controller if you're comfortable with controlling your monitoring volume via software instead of a knob. SPL makes a 5.1 surround controller for under $1k that works great.
https://spl.info/en/products/monitor.../overview.html
To get started without having a full surround speaker situation, check out some software called Spatial Sound Card.
http://www.newaudiotechnology.com/en...al-sound-card/
This works great to monitor surround in headphones. There are a lot of preset filters for popular headphones and the room spatialization is quite good.
Lastly, plugins: ReaSurround works great as a general surround panner.
I use Nugen Audio's Halo Upmix, Iosono's Anymix Pro Upmix/panner, and Exponential Audio's R2/Pheonixverb Surround plugs religiously too.
Ask me anything and I'll be glad to point you in the right direction!