I'm sorry I dont have an answer about weed and feed with guinea pigs but a freind wanted to use it on her horses paddock and she was told by the manufacturers that she should wait until all the granules had been absorbed into the ground and then wait 2 weeks before putting her horse back on it, however a science professor told her that the weed killer used can cause neurological problems if ingested in sufficient quantities and that it can stay on the grass until it grows out.
I also found this on a website 'Weed and Feed. Think again. Most weed and feed products contain pesticides, such as 2,4-D, dicamba and MCPP. Children's small size and developing brains and bodies leave them more vulnerable to the risks from pesticides.' (I know again this isnt Piggies)
On the Evergreen website it says to water in the pellets and allow the area to dry before letting children and pets on.
So basically I dont know.........sorry.:red