If you have not bought the bag yet check out the Camelbak Gambler. Looks like it could fit everything you need in a pack.
Yeah I already got the Heli pack and the 100oz Antidote. It fits perfectly and hangs nicely on the loop at the top on the inside compartment.
The standard mouthpiece does fit in the sleeve, but it's awkward to get out and put it back in, especially with my gloves on. I put the straight valve on and its really easy to use, although it does add a little bulk to the sleeve but it's not that noticeable so I don't mind.