Noob Bike Questions

1)  A compact crank is a double crank typically with 50 and 34 tooth rings (where a standard road crank has 53/39).  It gives you somewhat easier gearing across the range without the weight and fiddliness of a triple.  You can put one on most any road bike, although if you're replacing a triple you might need to change your front derailleur and shifter.  But with a 34 tooth small ring you'll probably lose some lower end compared to the small inner ring on your triple, so you might not want to.

2)  My aero bars (Profile Design Jammer GT) came with plastic shims that fit inside the clamps, so they'd fit a larger diameter (31.8mm) bar without the shims but fit a 26mm bar diameter with them.  I suspect your aero bars came without the shims.  You might be able to buy just the shims, though--check with the manufacturer of your bars.  Another possibility to try would be to try to put the clamps over one or two wraps of handlebar tape. 

As for width, narrower is generally more aero but wider is usually more comfortable.  You'll need to play with them (or get a professional fit) to find the width that works for you--which might interfere with the inline brakes.  I'd try them inside and outside the brakes first to see if either position works.  On mine you can also adjust the elbow pads in or out a bit.