well that's just it i truly couldn't find any answers to these questions

1. is it possible to give bonusses to a certain costume?
Yes it is, you can slot your costumes with Mysterious Chisels, you can then add KUSO (Costume) Stones to them.

2. wich bonusses are able to be given to them?
You can get certain stones that add +2/+4/+6 of a certain stat that you want, you can also add Crit Rate/M-Crit Rate or Crit Damage/M-Crit Damage to your weapon costumes.

3. to wich items could the "spotless" be given?
Spotless prefix can be put on your Weapons or Accessories. It cannot be added to Relics/Bows or Guns.

and i thinks that's it