If it were me...I'd be feeling a little tentative all night... a bit like when you park on double yellow lines coz you'll "only be 5 minutes"... but still hurrying as you never know...

If you want/need to pump out 600w RMS then using (but turning down) an amp thats capable of 900w RMS (ish) is being kinder to the amp - eg: its working in its "medium" capacity, rather than being driven hard ( or thrashed). Clearly this is what you're rightly concerned about and want to avoid - as if an amp goes "kapow!" it can take your speakers drivers down with it when it goes!

That said, I'd get a certain amount of re-assurance from knowing that the amps were QSC's, rather than some other brands.

Mind you...if you end up only needing to pump the speakers up to 500w RMS of output then your your question becomes "So is 750w rms amp (turned down to deliver 500w RMS into 600w RMS speaker gonna be fine?", and I'd be happy enough with that, especially on QSC amps.

Might be worth running a compressor/limiter 'tween mixer and amps, if nothing else, just to compensate for the all-too-natural speaker killer of your ears (threshold) getting used to the music levels over the first few hours, and slowly increasing the volume as the night progresses. On my own compressor/limiter (behringer MDX2600) its surprising to see that at times where I think that the music could do with a (subtle) increase, but a look at the compressor/limiter displays shows that the music levels are consistent with what they've been all night (crowd noise/atmosphere considering). Thats ear wax for you...