not a breeder but i have a little girl who might end up being a "tiny" ... she is almost 5 months and only 2lbs, 6 oz ... she has only gained about 7oz since i got her at the end of december...
my vet INSISTS on spreading out her shots/vac...sometimes she only gets 1 shot at a time and sometimes 2 at a time (depending on the shot) and they are always 3-4 weeks apart ... my vet said she is way too small to give more than 1-2 at a time and she worries about any negative reactions ... she always makes sure she tells me to watch her like a hawk the day she gets her shots and to make sure i call or bring her in if she has any reactions ...
not sure if this helps but hope it does
