If a guinea pig has soft poos, please take it off any fresh food (veg and grass) until the poos have firmed up again for 24-48 hours. Then reintroduce fresh food slowly and gradually.
Make sure that your piggies have unlimited hay and fresh water daily, which should make 80% of the daily food intake anyway. Veg (ca. 50g per piggy per day) should only make about 10-15 % and pellets (about 40g for young piggies, 10-20g for adults) should make 5-10% of the daily menu. Be careful when introducing any fresh food and grass, as tummies have to slowly adapt to them, especially in new guinea pigs that come from a background where no or very little fresh food has been given.
Recommendations For A Balanced General Guinea Pig Diet
Feeding Grass And Preparing Your Piggies For Lawn Time
If it gets worse or if the diarrhea is fluid, please see a vet promptly; asap as a life and death emergency if the runs are very fluid, very dark and very smell.