When I had to use it earlier this yr cause my mares foal got stuck and died in her, after removing the foal, my vet gave me the same stuff at 1mm (cc) every 1/2 hr. Then 2mm (cc) every 1/2 hr AFTER 3 hrs later if her plecenta still was'nt coming out.
My vet labels EVERYTHING, including what is in a syringe. Dont understand why they gave you this drug for a horse not cycling right. Perhaps give them a call to double check the dose and ask their thoughts on why oxytocin?
I too would question why Oxytocin. Regumate is what is usually used to regulate cycles (and it would be more than one dose); but perhaps your vet has another reason for giving the Oxy? But yes, most of the time mini's get the same dose of drugs and vaccines as the big horses.
The oxytocin might be the correct treatment, but it is obvious there is a communication error between you and your Vet. Get that cleared up before you give anything and likely the communication will be better in the future.