If these are not fear poops but the result of them not being used to veggies (wherever they came from), a bit of cooked rice along with what veggies you give them (giving them only a little bit at first) can get them used to fruits and veggies. Rats can eat quite a bit of veggies with no digestive issues, if they are used to it.
I would only worry if you start seeing liquid poops, soiled bums and / or poops that appear to have mucus in them. That might indicate that a more serious digestive issue is at play and a vet can give you benebac to start with and metronidazole (don't quote me on the spelling) if things don't resolve with just good bacteria.