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