I have both planes (32" w.s.) and find the Yak 55 smoother to fly. I use a 26-28 gram Blue Wonder for power (1300Kv).
I don't think a 19 gram motor will put out the wattage when equipted with a 9" prop...I could be wrong though. I use a 460 milliamp/hr 3s battery and get a around 10min. of flying time. I have tried a 360 milliamp/hour battery with success and found flight time to decrease proportionately (smaller fuel tank). A friend has used 9" props of varying pitches with a 2s 460milliamp/hr with great success doing hovers and knife edges etc.
You have to have the power (wattage) available with a 9" prop to hold the weight of the plane in strange positions (8-9oz AUW, 7-8 if you're really good) and overcome gravity but no more than necessary. To much extra power means unnecessary weight from the windings and motor stator.
Read Tallflyer's thread on the SuperSlo 28" build secrets in the Scratchbuilt section. The planes are close to the same size and the SuperSlo builders try to get the plane in at 7oz AUW. They started using the Blue Wonders from day one and the thread makes an interesting read for Yak and Edge builders looking for ways to cut weight.
Good luck! Both are excellent planes and Mike is great to deal with...go to your local Staples and pick up a sheet of white construction board to make templates of which ever plane you decide to buy. Never hurts to have a template when you want to build another one.