Is there missions that a UIT guy can do to improve both faction standing? Or a way to be respected/admired by both?

No can't be respect or admire with both of one likes you the other dislikes you. Could have just asked in game : /

+600 is the best you can get with either Serco or Itani. The sum of the two is no greater than 0 ro if you have +600 Serco you are going to be hate or -600 Itani