The organizers has not replied to my email about being able to use a flash. I am aiming for prints because if I am able to sell the images then I would like the quality to be there. I may rent a 24-70 L.
If you're renting, why not a f2.8 lens (24-70/70-200)? Maybe you can find out how far you'll be off to get the correct zoom range. Esp. if other pros around they might not even take you for serious (or even might get aggressive) if you've got gear like the amateur from next door. Otherwise the 50/1.8 & 85/1.8 with monopod to compensate for the missing IS sound ok (though the 50mm is not very sharp @open aperture), but if the models are moving you might have to crop (= even more noise).
I have been told to keep the Shutter speed between 1/200-500, F/stop 2.8-4.0,AI servo, spot metering, and Iso no more than 800 due to the noise and because the t3i does not do well in low light.
I also thought about trying something like this with my 60d, but really with the 18mp sensor & shooting for prints this is exactly what you need a ff body + f2.8 zooms for (= big $$$). You might be ok with web-size & iso3200, but other than that the shutter speed might get too low.
With flash & shutter above 1/250s you'll need to use hss (only possible on hot-shoe, not with built-in flash as controller) and your 430ex2 might not have enough power, esp. for multiple shots (that's what you need a 580ex-type with external power pack for). And did you try af servo with your camera before? It really sucks, you might be better off with one-shot when the models stop & pose. Spot metering is very dicy, too, because of the very limited dynamic range of the aps-c sensor esp. at high iso - so if the camera catches a too dark spot the highlights could blow.
But really, maybe I'm too pessimistic, let us know how it went!