As above, I would start Exum at SG and Rondo at PG, then bring AB off the bench as out sixth man.
Even in that role Bradley would never need to play PG. He can play SG alongside Rondo when Exum sits, then when Rondo sits Exum moved to PG and Bradley stays in at SG.
A backcourt of Rondo/Exum/Bradley has the potential to be the best defensive backcourt the league has seen for many, many years.
Everything I've seen of exum he has no clear advantage at either guard position - I think he prefers to play PG, but his skill set translates just as well at either position because he has great size for a SG, moves very well without the ball (off cuts, etc) and is an excellent finisher at the rim (either off the dribble of in the post).
This is why he reminds me so much of Penny. With Penny you could play him at SG or PG and itdidn't matter either way,he was equally effective at either position. Plus even at SG he still had a size advantage over manyopposing SG's
Like Exum Penny was very skinny and an explosive athlete when he came in to the league, and like Exum he had high IQ,
good handles, above average rebounding for a guardand legit PG skills. The resemblance is almost uncanny.