Hi Kabrakan,
I think that with both the 500 and 900 series kits that the pilot should consider 1/8" thick rubber times two loops or 4 stands. As for length i think 1.50 times the prop to rear motor peg length. I don't recommend double the length because the motor will "Grow" or strech out.
I think that the 900 series kits definitely fly better than the 500 series kits. I recomend using Peck Polymer props at 7 and 6 inch diameter props.
I now build the 900 series reduced to Peanut size models. It's been awhile since i built a full size 900 series kit. The video is suppose to show a full size 900 series kit crashing and the peanut model flying fine in a small baseball field.
https://youtu.be/E0bTMb7ErrA