Newbie needs help with setting up with a combo amp and through an interface for recording

1 - If your Randall combo is mono then I would use the monitor out with the output setting as Master Mono. Send this to the Randall FX return and tick Monitor Cab Off in the Kemper. You can leave this connected even if recording via the main outs and there is no need to change the Cabinet settings as Monitor Out and Main out are independent of each other.

2 - both options work well and have individual pros and cons. Which suits you best depends on your own workflow. For example, if you want to record a safety DI track for reamping at the same time as a mono amp track then SPDIF is excellent for simplicity. If you want to record Stereo amp tracks without a DI and your interface has good inputs then main outs L and R works great.

3 - I would leave the Cab on for recording but turn it off for your Randall combo.

4 - if turning off for use with a guitar amp I would use monitor output with Monitor Cab Off ticked in the Output Menu.

5 - see 4

6 - Best FRFR is subjective so only you can answer that. I personally really like my Kemper Kabinet but just remember that you will need a separate power amp unless you have a Powered Kemper.

Welcome, have fun here in the forum and with your Kemper. And yes, it is an incredible machine. Enjoy it as much as most of us here do

Alan answered your question perfectly, just to add on the FRFR: Depends a bit on what you want or what you use it for. Kabinet (or Kone as I use it in another box) would give you more of the "amp in the room feeling" for rehearsals and gigs. In parallel I am using a very compact dbx wedge monitor, not even sure how "F" it is. But good enough.

Use the search function of this forum and try to find lots of threads around Laney, Headrush, Yamaha and other common FRFRs that have been discussed quite frequently here.

Quote from Cannonball

When I am setting up, I take it I click on output, and then tick Monitor Cab Off, is this correct?

yes.

if you have a powered Kemper you could also try unplugging the speaker jack from the Randall and just plugging it straight into the Kemper Speaker Out.

I’m not familiar with the Randall RG200 but many combo amps don’t have the speaker hard wired to the amp. Instead they have a cable from the speaker with a 1/4” jack plug that connects to a speaker out on the amp itself. In this case the speaker can be unplugged from the amp and just plugged directly into the red speaker out of the Kemper and completely bypassing the Randall amp altogether. However, if the speaker in the Randall is hardwired and can’t be unplugged then you would use the Monitor Out of the Kemper into the FX return on the Randall. The signal from the Monitor Out and Speaker Out is the same except one is amplified to drive a speaker and the other isn’t.

Quote from Cannonball

It's really weird, I did what you said but I was just getting a flat normal guitar sound from all my profiles but when I ran left and right leads from the main output left and right to the send and return in the effects loop in the back of the amp, I was getting different tones.

In the output menu you can select which signal you send to each output. Make sure that you are sending Master Mono to the Monitor Output. It sounds as though you currently have it set for Git Analog pr Git Studio selected.

Alan covered all the bases really well, and disconnecting the Randall amp and plugging the Kemper directly into the speakers would be the way I went until you make a call on an FRFR / Kabinet, which would allow you to completely remove the Randall from the equation.

Regarding "best" FRFR, in addition to "best" being subjective, there are different ways of thinking as well. I haven't bought a Kabinet in part because it wouldn't fit in my small car (or GAS would surely have gotten the better of me). However, I actually like the fact that my stage monitor (DXR-10) is really just a small powered PA speaker.

My perspective (which I'm sure has many logical holes in it) is that if the crowd is going to hear me through PA speakers, that's what I want to hear on stage as well. I have absolutely no doubt that the Kabinet gives a more authentic amp in the room experience, but I want to get accustomed to what the crowd will hear and choose my tones (and expectations) to support that. Of course, once your guitar gets to FOH the sound guy can do who knows what to it, but in an apples to apples comparison that's why I'm happy with running a PA style FRFR for my monitor.