Starcraft Rules wrote:
Exception: If all of a player’s units in a given battle have the assist keyword, that player chooses one of those units to be his front-line unit. Such a battle will therefore have only one skirmish. The rest of that player’s assist units will be supporting units in that one skirmish.
You still play cards as normally, but the assist unit will of course use the minor values.
Also in the rules wrote:
7. Specialty Support Icon: This icon only appears
on Reinforcement cards and will always be found in conjunction with one (or more) of the Unit Icons on the card. The presence of this icon simply indicates that the special ability of this Reinforcement card
can be used if the portrayed unit is the front-line unit OR a supporting unit in that skirmish. For example, on the Reinforcement card shown above, the special ability may be used as long as its player
has a Wraith or Ghost unit present in the skirmish, either as the front-line unit or as a supporting unit
So your reinforcement card works as normal.