3-4wk old keets are substantially bigger than newly hatched baby chicks, and keets at any age are much more active than baby chicks are... so the chicks may get trampled or piled on, they may get kept from the feed and water, and they may get picked on right from the start, especially if you are planning on adding the chicks to the brooder the Guineas are already in. The Guineas may have already become territorial and protective of their own space and might not like the new additions, even tho the keets are just 3-4 wks old...
Brooding the keets and chicks side by side in the same brooder but separated by wire where they can constantly see each other but not get to each other until the chicks are older and bigger would be a better choice, IMO. And when/if you do integrate them all, I would put them into a completely new brooder or out in a wired off section in the coop that the keets haven't already become territorial/possessive of.
Hopefully you have a really big coop and pen for them all when everybody is older, or a 2nd coop and pen in the works because you may end up having to separate the Guineas from the chickens. Whether you have only laying hens without any roosters or a mix of hens and roosters... just 7 Guineas have a major potential to cause a lot of problems in the coop/pen, especially early Spring when breeding/laying season starts to kick in for the Guineas.
Best of luck.
