Skill Defaults Err? Also 40K flavour Qs

[sungnif]

I don't think I understand how skill defaults works in GURPS4e.

Diagnosis defaults to IQ-6 or First-Aid-8 (and other things). If I'm understanding that correctly, you can diagnose people at IQ-6 if you don't happen to have First-Aid. However, if you're trained in First-Aid, Diagnosing someone becomes drastically harder (-8 instead of -6).

Obviously I've got something backwards. I'd very much appreciate some help.

Also, I'm contemplating GURPS'ifying Dark Heresy, and it occurred to me I have no idea what various martial arts would be called in 40K. Now I might limit myself to just one called "martial arts" but.. I mean, since GURPS has a bunch built in, might as well use them no? So yeah, names of martial arts in 40K would be great, if you have them.