You have a situation to deal with don't you? Sometimes life is not easy.
Since you don’t know what stage the guinea eggs really are at but you think by candling they are not that far from your chicken eggs, I’d add them and treat them exactly like your chicken eggs. Stop turning and raise the humidity on the timing you would for your chicken eggs. Guinea eggs take 28 days, not 21, but what else do you have to go by? Chicken eggs don’t really need to be turned after about 14 days so maybe the guinea eggs are far enough along that turning is not a problem.
This should not jeopardize your chicken eggs and you might get some guineas out of it. One thing I’d consider, if you can manage it. You might want to physically separate the chicken eggs from the guinea eggs. Maybe build a fence out of hardware cloth or even an open-top cage out of hardware cloth and set that over one set of eggs, open top down. That will keep the first ones that hatch from crawling over the others and getting them all messy. That’s in case the guinea eggs are quite a bit behind.
When the chicks hatch, they will poop and create a mess in there. If the guinea eggs haven’t done anything by the time your chicken hatch is over, you might want to clean out the incubator and then put the guinea eggs back in there. Otherwise that wet poop and mess could start to smell.
I don’t see any real good solution. I think this gives you your best chance at success. Just be flexible and good luck.