I personally think using lesser used mons is good and I'm definitely someone who prefers that. The difficulty, assuming you are building a serious team to use on Cartridge, which I believe you are, is finding a niche for that pokemon to fit within the meta and within your team. A lot of lesser used mons aren't neccessarily bad, there are just often better alternatives available.Alright cool thanks for that. I've never looked at Discord but I'll keep it in mind. Forums are a slow way of interacting but make for good reading. I have checked out some of the usage stats and quite a few of those sample teams although I'm yet to use any of them extensively. I like to try lesser used mons if only to understand for myself why they don't get much use but perhaps I should re-evaluate that decision until I've become more familiar with standard teams.
Take Empoleon, which is 70th in usage in S13. I personally believe that it is an excellent pokemon in the meta. It both has a great defensive typing, a reasonable movepool and is very bulky. The problem with Empoleon is that you also have Hippowdon which is, in general, a better sr/phazer, Tapu Fini which is a better bulky water and Greninja which is a more reliable torrent nuker. What if my team needed a bulky water type that could check some forms of Mega Salamence but also could switch into Volcarona or Naganadel? What if my team needed a mon that could fulfill the same role as Hippowdon whilst beating Greninja and shutting down opposing Hippowdon?
Empoleon's your boy.
You then have to think, how important is that niche within the meta and how often you'd prefer Empoleon over Hippowdon or Tapu Fini within the context of your team composition. If you'd rather always have something else, it isn't fitting either within your team or the meta. That's the major problem with your Guzzlord, I can't think of a reason you'd ever want it over Hydreigon. There are pokemon though, even in the 100s and 200s in usage that have a niche.
The best way in my opinion is to build your team is to choose your pokemon based on what synergy you think they have, keeping the top 20-30 or so pokemon and their most common sets in mind, then you can fill out the movesets based on that. It's best for your pokemon to have a role within a composition, rather than thinking in terms of pure typing. Building FWG or FDS cores for the sake of having the type coverage is not very good unless those pokemon also have proper roles. As Chemcoop says, most of this just comes with time and experience. I cannot say that I am the most experienced with this myself, but keeping pokemon and their most common sets in mind will actually help you a lot I feel. You talk about stall and whilst it is cool to have an answer to stall, having a focus on beating stall when it is not a particularly common playstyle (or that good in general) is not so good.
I think after you build your team, bring it to showdown ladder and then start looking at team matchups and lead scenarios and tweaking your team accordingly. You can do this both theoretically and see it unfold in practice.
Take Koko-Aegi-Mence-Hippo cores which you are likely to find on the ladder, your team is actually somewhat naturally decent vs. them. Rotom-H is a pokemon that just naturally pressures that matchup a lot, most specifically with firium-z, but also to a lesser extent with scarf. If you were personally uncomfortable in the matchup, a bulky firium-z set on your rotom-h might be an adjustment you could make to shore up the matchup.
Overall it's just all up to practice and experience.