1) Leggings or Pants?
2) Is it worth to use footwear with different sole color or straight black boots fine?

Straight pants will better match actor model in Mocap Studio. Although leggins or skinny pants also work good usually. Just avoid loose, baggy clothes.

Different sole color may help to track feet pose in some situations, but usually similar dark colors work OK. Avoid shoes of light colors (sole as well).

Thank you.
I have one more question: Will my fat belly ruin mocap tracking too much with x6 c922 setup? :D

Actor model in Mocap Studio has adjustments for body proportions, including general body mass index and separate control for belly volume. You need to make it look as close to your appearance as possible before tracking. Then it should be mostly fine.