Very vague on the map description...
Instead of using the unused heros, use the heros that can be placed.
They are actually the same unit, so there isnt much a big difference.
If you are hitting the cannot create unit really fast when the map is empty, then instead of spamming all and any types of units right away, set a specific ammount of units a computer/player must have before another spawn is created.
Nobody likes to play a map with TOO many units causing the game to lag up. Pace yourself

btw, even if you get that condition to work (probably a mod for sc) ANYTHING that reaches 0% hp = dead
and again, your description of your map is too vague. Is it a madness map? defence map? etc...