The 900 is a fantastic amp, but it's only really necessary if you want to run 2 x 4 ohm cabs or 4 x 8 ohm cabs. If you want to use two 8 ohm cabs or just one cab of any impedance you would probably be better off with the ABM 500 which is smaller, lighter, and cheaper. The ABM 900 is exactly the same amp as the ABM 500, just with two 575 watt (@4 ohm) power modules. If you only used one cab with the 900, one of those power modules would be doing nothing at all.
As for overpowering the cabs, the amp will only deliver too much power to the cabs if you tell it to (i.e. you turn it up too high). Both of the cabs that you've mentioned should work fine.
I gigged an ABM 500 into a 4ohm 6x10 for years and believe me, it was plenty powerful! edit:
: Sorry, I missed the fact that you had already bought the amp! I'm sure you'll be happy with it, I was just saying that it might be overkill
You'll be glad you have it when you play arenas!