Guineas are much more fragile than chickens, their legs in particular. I would not tether.
I had a flock and my neighbor had a flock of equal size. Mixed sex all adults. When they crossed paths it was always a war never a merger.
Then I started a second flock to run on a different part of my property. When the birds where still juveniles the got hit by a predator and knocked down to 4 birds. They did wind up merging with her flock. Of My original flock I am down to only one and she hangs with the chickens and has not attempted to join the other flock.
So it could go either way as I see it. I would ask myself... How much do I really care if they merge?
Guineas, I feel, are of their best value when allowed to free range. So I'd would take the chance and if it failed I'd just get more and try again when they are adults.
I do think you would have better odds if your flock was larger.