Hallo, Ashely and welcome!!
We would need to know first off how tall your mares are and what sort of build, to give you an idea (and I never do limit my broodmares, if they wan to be fat they can be, so long as it is not life threatening, obviously) my stallion who is 28" and heavy built, weighs in at 80ks at the moment, which is too slim for my liking at this time of year. That is 176 lbs , BTW.
My mares are various sizes but one of my chubby ladies weighs 383 lbs, and stands 32", she is middle weight, not heavy not refined.
Obviously as they become more pregnant they become heavier and some of my mares will top out at around 400 lbs.
These mares get in foal no problem and have their foals no problem.
I do believe in allowing them to lose a little weight in the winter so they come into spring a bit slimmer and the actual weight on them by that time is all foal!!
As to the ultra sounding, no, it is not dangerous to have it done, but you do need someone who is experienced.
It sounds to me as if your Vet is giving the usual vet excuse of "If I can't do it it can't be done" which is a little misleading but understandable, and if your Vet is not happy to do it you may well be able to find someone else who can.
It can also be done externally, which obviously has no dangers, or you could opt for a blood test, which is not completely accurate as it depends on knowing how pregnant the mare is meant to be, or you can purchase a Wee Foal pregnancy test and do it yourself very reasonably but, again you do need to know how pregnant your mares are supposed to be.
I hope this helps a bit, but hang around, there will be plenty of people here to give you a wide range of advise!!
Pick the bits you like and ignore the rest!!!