Welcome to the world of RP! I hope you have fun with it no matter where you end up. To answer your questions to the best of my ability:
The tilt between individual RP and guild RP does exist, but it’s less significant than you might be envisioning - there are organized Alliance guilds on both WrA and MG, and many of them have BNet channels to double up with their presence. The primary driving thing between the two is that MG is almost ALWAYS crowded in the RP hotspots, while WrA will sometimes have some real downtimes.
Now, this can be a double-edged sword. More people is good, but it can also be a little overwhelming with the chatter. Sometimes, it’s hard to track the scroll, and you don’t really get that intimate cozy, tavern feel if the place is packed wall to wall. My recommendation to you would be to make an alt on both WrA and MG and check out the Blue Recluse in SW during the hours that you’re most likely going to be able to play. You don’t need to RP, just scope it out. If you can say to yourself “Yeah, I’ll be comfortable in this level of activity,” then that might be the option for you.
As for the guilds, as I mentioned, there’s plenty of guilds on both sides. RPers can sometimes be a little shy about actually inviting people to join them, though, so make sure that your RP addon has it listed that you’re looking for one to engage with. (Also, as a general rule, don’t join an RP guild with expectations of doing crazy endgame content. It just isn’t much of a priority for them. Not to say there aren’t a few that get into it, but time limitations sink in.)
For what it’s worth, I main on MG, and find not only the RP spots usually filled, but a fair amount of walkup RP while questing. Sometimes, though, I just need to chill out for a bit and I’ll hop to WrA. RPers in general aren’t really bothered by your level or your gear, so there’s no real reason you couldn’t just make the same character on both servers, maybe give them enough attention to have some flight speed, and just fish for RP like that. (And you can always party up with BNet friends if you want to hang out with a Moon Guard friend on WrA, for example.)
Anyways, that was a little rambly because it’s very late right now, but I hope that helped!