That's great news, that means that she should be able to handle this well.
There was a toxicity study done on female Beagles, and this supplement appears to be fairly safe in dogs.
Magnesium sulfate was given intravenously at dose levels of 75, 300 and 1200 mg/kg or 34, 136, and 545mgs per pound of body weight to female Beagles. No deaths were observed in any of the dose groups and the lethal dose level was estimated to be higher than 1200 mg/kg(545mg/lb). In the 545 mgs per pound group, they did see vomiting, decreased movement, staggering gait, laying down more and flushing of the skin of the ears and around the eyes. However, these signs disappeared gradually and the dogs returned to a normal about 1 hour after dosing.
While your pup is tiny she is healthy and the dose isn't that high, 27mgs per pound of body weight, so I think she may have mild nausea, but that should be it.
Please let me know if you have any further questions.