It doesn't surprise me that it was so "easy" to have Perk fit in with the Thunder.
It also doesn't surprise me that it is taking a while to have our new guys adjust: Kristic, a bench warmer before, and now being asked to be a spot starter and rotation player....as well as Green who was a starter and now asked to be a bench guy.
To me, it is much more easier to have a player play his role, one that he is used to than to have 2 guys play roles that they aren't used to and then still expect to produce and win games.
It's a lot to ask of for new players.
I agree with your point but I think you're misreading why Green/Krstic aren't fitting in.
Chemistry-wise, Perkins fits in because he's being asked to be the same guy he was in Boston, only more so. Perkins is like this guy from Band Of Brothers:

He's no major or general, but Sergeant Bull Randleman is good for two things..keepin the troops in line and killin Nazis. That's Perkins..he's a great supporting cast member, but he can't be 'the leader'.
Krstic and Green have never been leaders, and we're not asking them to be leaders here, so I don't see the issue.
Actual Roles:
Perkins fits in with OKC, because his role as you say, is the same, and even plays more into his skillset. He's a tough rebounder and defensive anchor. They're not asking him to slow-gather his way into their hearts, they're asking him to play good defense on the P&R, and take the burden off of Ibaka in the defensive paint so Ibaka can go running around blocking shots like he does.
In Boston, Jeff Green doesn't really have a 'role' beyond 'come off the bench, and..do something'. Doc has said when asked about how to use Green "I really haven't figured out how to use him yet", and even during a game, when green wasn't scoring or producting particularly well as a player, Doc said to him as he took him out "Whatever you're doing, keep doing it", as it was clear that Green had sparked the run the Celtics had gone on. Doc will figure him out, but hasn't yet.
Krstic started all 47 games he played in for the Thunder this year, so he was hardly a 'bench warmer' Krstic though was traded to us because he couldn't do some of the very things we're asking him to do (be a tough defensive anchor and rebound). In that regard, he's playing in a poor role as well.