Studio monitor speakers aren't paired unless they come in the same box. Whether you buy two at the same time or one from one retailer and another from a different retailer, one now and another in a few weeks, months or years time, doesn't matter; as long as that particular model is still being manufactured.
An audio interface is essentially a sound card, but aimed at music production. Some have more features than others and get more sophisticated the higher you go. They don't need to be used for music production purposes though. You can just connect studio monitor speakers and away you go. For those people who won't use an interface for music production but will want to use a microphone, then that needs to be taken into consideration, as I don't think you can connect any standard microphone that many use for, let's say gaming. I think you need to use a microphone aimed more at professional use. Just for use with studio monitor speakers, an interface is a great choice.
There are many different manufacturers of audio interface, but within each price point, many are likely as good as each other. Some do offer a wider range than others. Choice probably comes down to features and maybe brand preference.
The Roland Duo can use one of three power sources; mains, USB or batteries. Doesn't come with the mains adaptor, so you can use USB. Doesn't have the nice satisfying large volume knob of the Focusrite though. There is a Roland Rubix 22, which does have a larger volume knob; although not as large as the Focusrite.
As for connection type: RCA is also known as phono, which is an unbalanced connection. TRS stands for tip ring sleeve. That's what 3.5mm and 6.35mm jack's are and can be used for balanced and unbalanced.
Unbalanced has two wires per channel; left and ground, right and ground; so most headphones and HiFi speakers.
Balanced has three wires per channel, commonly known as hot, cold and ground. The idea is that interference is more easily rejected than it is with unbalanced.
You'll need two 6.35mm to 6.35mm stereo cables.
http://www.gear4music.com/G4M/Stereo-Jack-Stereo-Jack-Cable-1m/10AN
Doesn't have to be that one specifically, but that one will do the job fine. There's longer lengths if you require.