Question about some of the items I’ve found playing more recently

Growing up I watched the first Gen of Pokémon and I played the first gen games as well has had the Pokédex for the first and second gen. I’ve recently gotten into watching the first gen series and plan on watching it all as well as the 2nd gen (which I’ve never seen). But from playing this game and kind of keeping up with the games (not playing them) I get confused about gen 3 and on. It just seems like a lot to keep up with all 500+ Pokémon and their types and whatnot. Is it worth watching beyond gen 2?

Now onto actual Go questions.

1. If I have a strong first evolution should I power it up first or evolve it to the last form and then work on powering up the last form?
2. I have a sun stone and for one of the research quest things I have to use it to evolve a gloom which I have, but it wont let me. I don’t have the candies for it but idk if it’s because I need candies as well as the stone or if I can just use the stone so do I need candies for it or do or do I need to do something else to use it? I also have a few metal coats and sinnoh stone it won’t let me use either.
3. I have a lot of fast tms. What are they exactly and what is it best to use it on generally?
4. Is the fastest way to get candies for Pokémon you don’t see a lot of is by using it as your buddy for a while and walking with it? I have a dragonair that has some pretty high IVs (three stars when I hit appraise) and I’ve had it for weeks and I feel like it is taking forever to get the candies to evolve it. Is there a faster way to get candies for it?
5. I have a lot of lucky eggs and incense. Is best way to get exp to level up to camp in one spot and use both?
6. Why is it that gyms look different sometimes? For instance near where I work there are three fairly close and one will have a thick tower type of thing to it and the others don’t making them shorter, all taken over by the same team. Why is this, does it mean anything really?

Sorry if these are really obvious answers, I’ve played Pokémon Go since it was released and quit after the first few weeks and played off and on extremely casually over the last few years. I’m excited to get back into Pokémon, reliving some nostalgia and learning about a lot of these newer gens!