Horses going from gr hay to a little bit of pasture? Plz answer!?

Ok so when we got our horses yesterday (two minis) the lady dropped them off and said that they would be alright eating a little bit of gr and weeds that are in their paddock that need to be gone...I have been letting them eat it but I was very worried that they would get colic or a sickness from being on just gr hay to a little bit of pasture hay...Now they are still eating it but not as much...She also said that they will stop when they are full, but when I take them for a short walk they try to eat...now their poop is a little bit runny but not super bad its just not firm and I'm worried that they are starting to get colic...We werent prepared becuase they came a day early and I have no thermometor and I cant put them inside the barn for the day becuase the older one will most likey kick the smaller one and hurt her....Do you think that I should do somthing aboout this??