I am starting this out by saying I get matched with people who tend to play their mains and only their mains, how do I exactly counter a bad team set-up?
We, for starters, need a tank, AD, AP, and good jungle capability. I got that down, what comes after that?
Their are many different team comps. Siege comps, poke comps, protect the adc comps, pick comps, skirmish comps, and more. In ranked, it is usually best to go with a good skirmish comp. A comp that has heavy AP damage from at least one champion, heavy AD damage from at least one champion, heavy aoe cc (from more than one source at least), and tanks/fighters that can survive for a long time in fights. IMO that is the best GENERAL comp to have.
I am starting this out by saying I get matched with people who tend to play their mains and only their mains, how do I exactly counter a bad team set-up? We, for starters, need a tank, AD, AP, and good jungle capability. I got that down, what comes after that?
If "we need a tank" then you are that tank. Otherwise you don't actually believe it enough to do it. Which begins to illustrate the point made about mains: Be capable of taking every role that "we" need, rank them in importance and then select the two roles most under-filled in your match history.
Yeah its tricky when people pick champions that pull the team comp in different play styles but at the same time its probably better they are playing a champion they are comfortable with.
So in the end its up to you to try to plug holes in a team comp with versatility.
If you are ADC, where you don't have many option as far as going tank goes. it depends on why you need a tank.
If your team need initiation but your team still has lots of CC then picking 
works, you start the fight then that allows your bruisers into the back line.
If you just strait up lack CC then perhaps you want someone like 
and 
he is self-sufficient and mobile.
and 
have long range and self peel.
and 
does everything he needs on his own.
Pick someone who fulfills all of the roles. Generalists.
Worst case, pick as much AOE as possible. If you can land a 5- man malphite ulti, or orianna ulti, you can win most fights.
