Not sure I understand why 50 cubesats in orbit for 3 - 6 months doing atmospheric research will 'cause stress' on bandwidth.
If traditional modulation techniques were used and you wished to have unique downlink channels (minimum 50 kHz channel spacing on 435 due to Doppler) you could end up using 2.5 MHz for the downlinks from the 50 CubeSats.
Hopefully other approaches will be used to reduce the bandwidth.
But remember this is on top of existing activity. 2013 is likely to see 30-40 CubeSats launched and it wouldn't surprise me to see even more in 2014, so by the time the QB50 deployment takes place they'll be a lot of activity already up there.
73 Trevor M5AKA