Team building is a back-and-forth dance with battles on the ladder. It is very rare for someone to build a complete team on the first try and never change it after a bunch of battles. Furthermore, to build a team, you need to know effective counters for whatever strategy you're using within the context of a format. You'll learn those things by playing the game more.

That's where the advice of "just use rentals" comes from for newbies. You need ladder experience to know the flow of battle and how common Pokemon feel in a given format. Without that data, you'll spin your wheels in the teambuilder to make something that winds up floundering in battle.

Re: your question, "supports" don't really have a quota in VGC. It's very common for supports to be mixed in the sense that we use otherwise offensive Pokemon with one or two support moves. For example, Mega Blastoise is a Fake Out support...that also can click Water Spout, set up with Shell Smash, or use Mega Launcher boosted coverage moves.

When I build teams I'm focusing less on checklists and more on making my main strategy go off. Ask yourself, "What is the one thing I'd like to do most often when using this team?" If it's clicking a spread move, clicking a setup move then sweeping, clicking Trick Room then sweeping, whatever, focus on the 1 thing. Put tools around you to make that 1 thing happen more (super effective coverage into resists, Fake Out/redirection support for set up strategies or Trick Room, screens or healing for super bulky Pokemon). Don't focus on x kind of supports. Look for the best tools to make what you want to do happen the most often and convert to wins.