Blimey, clear as mud there...

Here you go. My bias is towards Irish traditional, but what I say would apply widely. I'm talking here about 10-hole diatonic harps. Harp positions in brackets.

D harp: D major(1), A Mixolydian (2), E Dorian (3),B Aeolian (4), G Lydian/G major hexatonic (12).

G harp: G major (1), D Mixolydian (2), A Dorian (3), E Aeolian (4), C Lydian/C major hexatonic (12).

For those 12th position tunes you could nearly always play in first position on a harp in the key of the tune (in the above examples, a G harp and a C harp respectively), though it's nice to use 12th position if you want to make up sets with key-changes in them.
Note that what is referred to as "cross-harp" is actually second position, so if you play an A polka on a D harp you are playing cross-harp. Most of us just do it without thinking about modes or positions. Another point is that Lydian hardly ever arises in Irish tunes but I kept that in because there are a few 12th position tunes that are not exactly Lydian but which are great to include for the possibilities of key-changes in sets.