I use ScourGard on my cows a month before calving... 20 years ago we had 6 calves with scours at a time, all half dead, all getting tube fed (lost a number of them), and we started using the vaccine, never a problem since... If some cows don't cooperate, I don't vaccinate them (I'll try harder the next year though), and if I'm short of vaccine, I'll give them 3/4 or a dose... I usually give my heiferettes a shot of it at a year old, and don't give boosters to the first timers... I have had no problems doing it this way. We've never tried Guardian (if it ain't broke, don't fix it)
your heifers sound like they're about the weight of mine... Last year they were about 1100 at breeding, and this year I think they will be about 50-100 lbs short of that on average... One is in the high 900's now, and the others are right around 900... I figure a bit of grass should get them going. I've also found that as soon as they're bred, they stop (pretty much) growing.. must be the change in hormones