The mayor in the first port has a dialogue-option requireing the skill History 2. My main don't have this skill, but my companion does have History 2. However the option is still greyed out.

 

Is there a way to make use of companions skills/stats in dialogue, or is it all dependant on my mains?

I think companion skills count, but not 1:1 ratio like

The ratio goes like this:

1=1

2=2

4=3

7=4

11=5

And so on.

(Combined companion skill on left, bonus on right)

 

A companion with history 2 should allow you to pass a history check 2 if the companion is close enough. Some check cannot be assisted by companions though.