Well, if you want "the new ones", you will have to go the custom value way...
(or something similar)
There's no way to tell when exactly a unit came into play, unless you remember it somehow.
If random is enough, well, something like this:
Code:
Food
Events
Player - Player 1 (Red)'s Food used becomes Greater than 0.00
Conditions
((Triggering player) Food used) Greater than ((Triggering player) Food cap)
Actions
Unit - Kill (Random unit from (Units owned by (Triggering player) matching (((Matching unit) is A Hero) Equal to False)))
(If you need more players, add some more events)
Another way of going by "custom value" would be,
when you get a new unit, assign it some custom value,
add 1 to the global variable that holds the last assigned custom value,
and, when a unit dies, kill them off one by one, starting from the largest custom value.