I really wouldn't worry about cracks in firebrick. There are different kinds of firebrick, different insulating qualities and durabilities. a good-quality high or medium duty low insulating (dense) vitrified alumina firebrick will hold up the best. However, this is not always what the stove manufacturers recommend or use, and it's almost impossible to find good-quality firebricks of the type that I mentioned in a stove shop. Many stove manufacturers use less expensive lower quality higher insulating brick (just because a brick is higher insulating or lower duty doesn't necessarily mean that it's automatically lower quality). Regardless of quality, high duty dense firebrick will outlast low duty brick by decades. In my stoves, I have Evens-Howard http://en.wikipedia.org/wiki/Evens_%26_Howard_Fire_Brick_Co. firebrick made in the late 1880's from a defunct coal-fired boiler that's still in excellent shape and hasn't cracked (having been taken out of service, piled up, rained on, snowed on etc. for years before being put back into service) after 120 years!
Did I get poor quality firebricks?__________Possibly, but more likely it's just the nature of low duty brick- it can't handle the thermal changes as well
Do they need room for expansion and were they too tightly installed?__________possible, but unlikely that this had any significant effect
What could be the cause for these firebrick failures?___________either the low duty split is prone to cracking, or you have especially low quality low duty brick
What is the best way to trim fire bricks?_________Diamond saw blade on a cheap (you'll ruin the bearings) skillsaw both available at lows under $50