I'm going to implement Slicemachine to our Repository that was created a couple of years ago. I've created a new next app and pointing it to my legacy repo.
I've successfully added a slice with Slicemachine that I can now use in my existing repo.
Is this the correct way to "activate" Slicemachine to an old repo?
The only issue I've seen is that when I open a custom type and see the legacy slices, I get "This custom type contains slices that are incompatible".
So if I just don't edit those in slicemachine, I can add my newly created slices (created with slicemachine) to my existing custom types in my Prismic repo? :)
Thanks for reaching out 
Have you considered using our upgrade tool to transition from the legacy slices to the slice machine? It should make this a lot easier for you: Upgrade Your Prismic Project with Our New Automated Tool. There's more information on this in this forum thread: Legacy Slices to Slice Machine - #8 by d.mccormick.
That's correct. You can mix both legacy slices and slice machine slices.
Let me know if you have any other questions