From all the documentation I have reviewed there is no definitive definition on your enquiry.
If the payroll software you’re currently using is going to support STP as of the first of July 2018 then my advice would be to start reporting regardless of how many staff you have.
Most of the work to get STP compliant is done by the payroll software provider, from the employer’s perspective there is very little to do except click a few buttons.
If the payroll provider has applied for an STP deferral then your decision has been made for you.
I believe the reason the ATO consider multiple companies owned by a single director as a single entity is because companies were deliberately breaking up to avoid payroll tax, tax avoidance is far more serious than a grey area in a reporting requirement, thus whatever you decide will likely not result in fines, possibly just some emails from an automated system.
The rules will become stricter as more payroll software companies become compliant.
STP Business Scenarios v3.0