>> can I find rocks big enough?
Um... yes. It's the Grand Canyon. More big rocks than you can imagine. :)
My suggestion is to check the NOAA forecast for the inner canyon (google "NOAA forecast Phantom Ranch" and you'll get the right coordinates) a few days before your trip. Take a tent if they're forecasting any major rain (very rare.)
>> Would I have to bring a warmer sleeping bag?
Do you have any higher-elevation camps in your itinerary, say above 4,500 feet? If not, you won't even need your Helium.
I hike in the GC March/April or Oct/Nov. I've never taken anything warmer than a 30* bag. I often take my 45*. The coldest nights I've had were around 34*, cowboy camping on Whites Butte, which is probably around 4,500 ft (top of the redwall) and a snowy night on Horseshoe Mesa at 4,900 ft, under a tarp. Both were in mid-April. I had (a true) 30* degree bag for those nights and were fine. Note that average lows at the river is 48* for March.
ETA: Someone who hiked out yesterday 3/23 from doing the Escalante just reported mid-70's during the day and mid-to-upper 40's at night at the river.
Again, check the forecast right before the hike. We're currently planning on a 30* and a 25*.
You really don't need a lot of gear in the Canyon. Just lots of water. :)