Yes mites are bad. it helps to keep the ground dry and have an area of damp sand so if they do get mites they can clean themselves using the sand.
Does Collembola are compatible with Harpegnathos venator to help to don't have mites?
Yes they are compatible. Harpegnathos have often a chamber with different kinds of small insects like Collembola or Isopods.
Thank you Chemitech ;)
A video for illustrate the "doggy style" before fight:
http://www.youtube.com/watch?v=dXKxkSKWme0
edit: it does not work youtube link?
http://www.youtube.com/watch?v=dXKxkSKWme0