I've flown many Embraer RJ-145s, which also have the 2+1 configuration and similarly small overheads. However, my Storm im2500 will fit under the pair of seats. Not sure if that's true of the CRJ-700.
My advice would be to plan for the need to check your gear. Even on regular planes with normal overhead bins, these days there are fewer flights, they run fuller, and unless you've got priority boarding (which I do on a few airlines, fortunately) you run the risk of the overheads being full when you board.
A hard case is definitely the way to go - Storm im2500 or Pelican 1510. I prefer the Storm cases (the line was bought by Peli, actually) because the latches are easier to operate than Peli's infamous 'knucklebusters'. The problem with a hard case is that whether you use the foam for maximum protection or the dividers for convenience of reorganizing what you bring, you're still left with the problem of needing another bag to transport your stuff at your destination. My solution is to use neither the foam nor the dividers. Instead, I pack my gear into a suitable backpack, then put that backpack into the Storm im2500. The Lowepro Flipside 300 fits easily, the Flipside 400 AW and DSLR Video Fastpack 250 AW can be put inside with a tight fit. I use a pair of TamperSeal TSA SearchAlert locks (the long hasp version), and just lock the case if I need to check it.