No high torque servos will not ruin the plane. Just the opposite. It may even help prevent flutter which could cause a crash. THe high torque servos make it easier to hold KE and hover, etc. I always use them on my planes. On the small DO it isn't really necessary but it certainly won't hurt anything.

I use a 1200mA minimum battery. Most times I use 1650mA. I think 600mA is too small. You would be alright if you charge every two flights but you wouldn't want to go much longer.

Thanks
Barry