My guineas that I hatched were with my chickens from about a month on....right now the guineas are about 5 months and are scared of the chickens, esp the roosters. the chickens chase them off food bowls which is the opposite of what i read would happen...l had to put them in separate coops to keep the peace...If you start with keets, they will be fine together. When it comes time to introduce, they need to be penned in an area until the chickens have adjusted to them.
Keets are very tiny until they are several months old, so if the hens are a dominant breed, they could harm them.
Usually by 4-6 months or so, they defend themselves just fine tho!