If I wanted to make a knife for friends in the guard, is there anyting special it should have ? Lenght of blade, steel butt cap, black blade and handle ? What about the scabbard ? Does the guard frown on custom knives ?

Since Guard units are controlled at the state level, each unit must follow the state laws concerning blade length and such. I personally carry one of my EBKs and a Progression II folder on me all the time when in uniform.
When being deployed, the regulations state that "no member may transport with them any personal firearms, knives, or explosives, if a member is found to have in his/her possession any such restricted item, it will be confiscated, and the member disiplined."
That is not to say that each of us doesn't "pack" something away for emergencies. When I deploy, I hide a large kukri in my ruck sack, and then when I get where I'm going, I have the knife set up where it attaches with snaps to the outside of my ruck sack for easy access. They tend to give me a bit more leeway than most, only because I'm in charge of all the weapons and security for our outfit.
Most of the time they will say nothing if the knife is no larger than the standard issue Air Force survival knife.

Ed, glad to see your still at home. Calif. is crazy with knife laws. Maybe I'll just make them a nice hunter instead.