This one took some research, but I think the answer is:
"I began as an expression of warning.":
4, which sounds like "Fore!" as in golf
"At 2, my opposite brought two scales together.":
40 - The opposite of 40 is -40. -40 Celsius = -40 Fahrenheit, two temperature scales.
"At 3, saw the death of Epiphanius of Salamis":
Epiphanius of Salamis died in 403, so you could say that the year 403 saw his death.
"At 4, was widely called weird":
4030 is a weird number
"At 5, grouped online with HDMI cables":
Googling 40304 returns lots of HDMI Cable links
"At 6, seen with Sigma-Aldrich's tannic acid":
403040 is the product number for Sigma-Aldrich tannic acid
"After much inactivity, at 9, I was associated with a Saudi oil tanker.":
403040000 After much inactivity (just more zeros), associated with a Saudi Oil tanker - 403040000 is the Martime Mobile Service Identity (MMSI Number) for an oil tanker from Saudi Arabia (called Fajr). An MMSI starting with 403 designates a Saudi vessel.