If you are using dual deploy I would go with the 54" main. However, I think you would be OK using the 36" chute. Several times I have launched a 2 kg rocket on an 18" drogue and a 36" main. The rocket landed softly on the two chutes. It also landed safely once when the main 36" failed to deploy and the rocket landed only on the 18" drogue. The descent speed was much higher than I would have liked on just the drogue, but the rocket was fine. It landed in a plowed farm field.
NAR# 92125 L1
Total Impulse for 2011: 1,729 N/s
Total Impulse for 2012: 1,689 N/s
Total Impulse for 2013: 795 N/s
A:6, B:5, C:19 D:15, E:4 F:0, G:0, H:1, I:0