We don't have a big man on our roster capable of rim protection. If defense is the goal, I guess Sully + Bass is probably the best option. That's still pretty weak, though.
I'd start KO at center and Sully at PF. There will be lots of growing pains, but that's the stage we're in.
I actually have an optimistic outlook of a KO/Sully frontcourt defensively.
Yes, there will be no rim protection, but I think rim protection can be a bit overrated at times. If you want an elite defense, sure, you need it, but you can still have a decent defense without it.
The biggest worry I would have with that combination would be having either of those guys chasing quicker guys on the perimeter. That would be an issue, but you can always put Wallace, Green, or Bass at PF when you face matchups like that.
But, I actually like both Sully and Kelly's potential as pick and roll defenders. When it comes to defending the pick and roll, footspeed isn't nearly as important as intelligence. It is all about position and angles. By the time Sully got hurt last year, he was the best pick and roll defensive big man other than KG on the team...and by a decent margin IMO (granted, he didn't have a ton of competition), and I think Kelly has the same potential.
If they can really be precise about their positioning, and make smart decisions reading plays, then they both could be very good pick and roll defenders, which would go a long way. And then when you have ball-hawks like Rondo, Bradley, and Wallace around causing havoc, you can really get by pretty decently without having that interior presence defensively against most teams (and in the regular season...it would be a very different story in the playoffs).