As far as the mat-burn on toes goes, I can give a little bit of advice. Keep your toes curled up. I've seen some people doing shikko-waza with their feet flat - this'll give you nasty mat burns on your toes or even the top of your feet. I had this problem before I was corrected.
The same also applies to rolls. Keep your toes up as you're coming out of them, or get nasty burns on the tops of feet and toes.
If it helps to explain better, shikko-waza is done more from a kida position, not seiza.