You have 2 different commands you can mix.

I think what you want to add to a unit is the following:

rank -1 #sets back rank
rearpos #sets unit to rear most position within the rank

For overview of positions see http://www.illwinter.com/coe4/coe4modding.pdf (pg 11)

rank <value>
Sets the rank for the monster.
-1 = back, 0 = mid, 1 = front

Then you my additionally add one of the two:

rearpos: This unit will place itself as far back from the front
row as its rank allows (e.g. pikemen will always
be at the rearmost rows of the front ranks).

or

frontpos: The unit will always place itself in the front row, or
as close to the front row as possible if there are
more rows.