Kog'Maw is actually more heavily prioritized by professional teams than Tristana. Also, Kog'Maw is noted as being very safe as an ADC; he's strong early, mid, and late game, something no other character has. So long as the player knows how to use Kog'Maw, they don't need their entire team to revolve around them. Tristana actually needs more help from her team early since she is weaker; if she doesn't reach her core items, she doesn't provide anything to her team, where Kog'Maw still has a lot of utility in his slow, his ult (free vision), and the resistance shred on his Q.
As for the team compositions you suggested; Lulu is a lane bully, arguably a better one than Renekton. Assuming the teams are equally skilled, Lulu will most likely win that match up, negating Renekton's strongest point (his early game). Lee Sin actually fares really well against Kha'Zix, since Lee Sin has two methods of revealing stealthed champions. Lee Sin's ganks are also more powerful, especially since Kha'Zix is less likely to gank early since he can farm and be a late game carry, which generally means Lee Sin can snowball his team. Lee Sin also has great synergy with Orianna, since he can place the Ball very well for a shockwave. Orianna vs Syndra is most likely just a skill match up, but the emphasis on defense that Orianna's team has (shields from Lulu, Ori, Lee Sin, and heals from Lulu ult, and Nami) means that everyone on that team is harder to kill, not just Kog'Maw, meaning it's very hard for Syndra to burst someone with ult (or Kha'Zix/Tristana to assassinate someone). The bot lane match up is a bit harder, since Braum counters Nami in that he can block her ult with his shield, but at the same time, Kog'Maw doesn't have a weak point in the game; he's incredibly potent at all stages, where Tristana has a notoriously weak mid game. Not to mention, Team A has insanely strong disengage potential, with Lulu and Orianna having incredible utility, Lee Sin's kick, Kog'Maw's slow, and Nami's ult/bubble. Team B's only engage tools are Syndra stun or Braum ult, unless they get the upper hand on vision and are flanking, etc. which is incredibly difficult given there's a Lee Sin on the other team who's probably carrying a sightstone, in addition to the support.
You can argue that Kog'Maw's team loses damage if they create a composition revolving around protecting him, but there's a lot more to winning a game than raw damage; more-over, ability to use that damage is even more important, which Kog'Maw can since he has such an enormous range advantage over most other champions; if Team B builds purely for damage, they lack many options when it comes to engaging and actually using that damage; brute-forcing isn't going to work particularly well against a team that is so good at disengaging and kiting.
