Slitherine

Mixed Battle Group
"H" = 4 HW / MF
"C" = 4 CrossBow / MF

"O" = some other enemy Battle Group
"X" = spacer just ignore this
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OOOO
HHHH <-----front
CCCC <-----2nd rank

Question is if the Mixed BG loses a stand in Impact how would the the rest of the unit fill in the gap in the front rank?
We played it that the Crossbowmen would move up.

OOOO
HHHC <-----front
CCC <-----2nd rank

How is future melee resovled?
We played it that the "C" (crossbowmen) that is now in the front rank has to have his own POI different from the "H" (heavy weapon dudes).

Also would I have to lose a stand in front base to base from Impact?

OOO
CHHH <-----front
XCCC <-----2nd rank

If this is so, Mixed Battle Groups are painful to use. =)

Yes, it looks like you played it right. The only slight difference is that, although losses have to be taken from the bases in contact, their replacement can come from any of the non front rank bases. In your example there isn't any benefit in this, but if the BG were like this (facing up the page):

H H H
C C C
H C

you would lose one of the front rank H bases, but you could use the one in the rear rank to replace it.