Are pipe unions made to a standard size / threading?
As far as unions from different mfg's of different sizes being compatable my experience is no they are not, even the same brand I checked going from 3/4 to 1" would not mate up I was able to find one that was 3/4 on 1 side and 1" on the other but next time I will probably just use a reducing fitting as the "special" union was more expensive than a standard 1" union and a nipple + a reducer but I had a space limited amount of room to make the repair so in that case I paid the extra. Also make sure if going from copper to galvanized to use a dielectric union. The failure of a home owner to do this in the past has caused plumbing failures in under a year in past repair jobs.