I'm not wild about John Collins fit-wise.
(1) He is a score first / non passer type of offensive player which is not a great fit next to Jaylen and Tatum.
(2) Collins is at his most lethal when the paint is open for him to use his quickness & athleticism going to the basket which means he works best next to a stretch five rather than a paint dweller like Rob Williams. I do like Collins fit next to Horford at center. Next to Rob, Collins becomes an overpaid ordinary player who gets unhappy with his lack of offensive involvement just like he has been in Atlanta next to Capela. I would trade Rob Williams for a long term stretch five (say Myles Turner) if we acquire John Collins.
Collins has a basic perimeter game to be functional next to a paint dweller at center. But I do mean basic & I do mean functional. He can exist but he cannot flourish. His 3PT attempts are low. His handles are limited. His passing is limited.
Where Collins flourishes is in PnRs, cuts, offensive rebounds, isos from the midpost and low post post ups. He needs to be in and around the paint to flourish. In other words, he needs to be in and around the paint in order to justify a big payday & to keep him happy otherwise he becomes an unhappy guy in the locker room.