I think he wants some control over what allies are in the list to use as helper squad.
And I'm not sure there is. Last log-in time seems to have some effect on the list, but there also seems to be some kind of "cooldown timer" for a helper squad after you used it, making it not appear again for about 15-30 minutes.
So, your best bet to get a specific helper squad, would be doing a couple of 1-1 runs (lowest stamina, skip any SG that might appear) until your desired helper comes on the list, and then do the map where you want to use that specific helper.