You can put a 3" adapter on the PC but the rupes mini is far far superior and works great. I actually have done the adapters on the PC and found that the connector gets loose real quick which may result in the backing plate flying off. Very risky to say the least.
The Flex definitely vibrates more than the Boss, especially on speed 6. At that speed you can't control it so I only run it at 5 max and that is pushing it. As I stated, it depends on what is more important; if you are doing this all day every day vibration really matters but if a weekend warrior I found the time savings with the flex to be so advantageous the vibration is secondary. I also found that vibration gloves really help reduce any strain as well as ear plugs. I think your decision relates to vibration vs stalling stopping (although a lot of people maintain the Boss/Rupes with proper technique doesn't stall, which wasn't the case for me).