Tomo did an interview with Noel on the review of the Hydroshort and Noel asked a similar question.
Tomo explained it as the Cymatic and Hydroshort being in 2 different categories in his mind. The Cymatic is more of a Planning Hull design and is supposed to be very parallel in the outline, and have a wide squared off nose and ridden as short as possible. The Hydroshort is supposed to bridge the gap between a traditional high performance design but with softer aesthetics - specifically in the nose.
So in my opinion the Modern Planning Hull boards (GMM2, Evo, Vader, Vanguard, Nano, Cymatic, etc.) can be ridden undervolumed, because of their design, but the Hydroshort is meant to be ridden more true to your normal shortboard size and volume.
I think that 5'9 sounds like it would work for you? Im 5'6 - 150lbs and i ride both the stock 5'5 and 5'6 which are both in my liter range.