In some respects you are better off having a bit more power in the amps than the speaker are rated at, it more easyer to blow a speaker when an anp is being worked to the max even if is rated power is lower than the speakers.
You have to be carefull though, your best bet is set up a compressor limiter on the system, providing it has been set up right it should be near on impossilble to blow anything.
By having more power in the amps than what you need means that the amps are not running hard or at there max so they shouldn't overheat or distort, but agian you have to be carefull !!
There is no true way of finding out how much power you are sticking into a speaker ( As far as I know ), the best way is when you are setting up & doing a sound check turn the sytem up until the point where its on the edge of sounding rough & distorted, then turn it down a little & set the limiter to that point. Thats what I have done when I haven't known the power on the speakers & amps ect that ive been using.

Sorry to ramble on abit but I hope it helps :D