I just bought a 2611 a month or so ago so I'll share my findings. As stated above 6.5" pads work fine out of the box. You're limited on changing backing plates though. The backing plate on the 2611 has 4 mounting points as opposed to the 1 used by the PC. Wannafbody found a 5" Ryobi backing plate at Home Depot that uses 4 mounting points and I saw the same one at my local store. I compared it to the display model 2611 and it looks like it would fit. Now, I'm not sure I would try it with the 2611 as there's a guard/casing that surrounds the stock backing plate. With a smaller backing plate installed, you could possibly hit/scrape your car with that guard if you're not careful. It may work fine too...I don't know.
Also regarding the warranty, don't know if you've looked into this already, but Ridgid's lifetime warranty only covers defects in labor or materials and does not cover standard wear and tear. So, although "lifetime" sounds impressive, don't think that you'll never have to pay for repairs down the road. Additionally, you either have to send it in to Ridgid for service or take it to one of their authorized repair centers. Based on some posts in the Ridgid forums, Home Depot won't do anything for you once you've owned it for 30 days or more. In any case, I would spend some time on the Ridgid site if you haven't already. The complete warranty is posted and you can look up the authorized service centers in your area. The 2611 manual is also available there too. (I personally found it useful when trying to decide what to buy)
In any case, hope this helps make a more informed decision.
Also regarding the warranty, don't know if you've looked into this already, but Ridgid's lifetime warranty only covers defects in labor or materials and does not cover standard wear and tear. So, although "lifetime" sounds impressive, don't think that you'll never have to pay for repairs down the road. Additionally, you either have to send it in to Ridgid for service or take it to one of their authorized repair centers. Based on some posts in the Ridgid forums, Home Depot won't do anything for you once you've owned it for 30 days or more. In any case, I would spend some time on the Ridgid site if you haven't already. The complete warranty is posted and you can look up the authorized service centers in your area. The 2611 manual is also available there too. (I personally found it useful when trying to decide what to buy)
In any case, hope this helps make a more informed decision.