Absolutely, this can vary. IME at least in the old days, it did tend to be a single unit type (eg Red Alert Medium/Heavy tank rushes, or a well timed Zerg Rush)

Absolutely, this can vary. IME at least in the old days, it did tend to be a single unit type (eg Red Alert Medium/Heavy tank rushes, or a well timed Zerg Rush) but it is by no means a hard and fast rule.