I’m not sure exactly what you mean by leaf spine in this setting, but we have MAAS running DNS, DHCP and machine management for a subset of our machines, in a data centre which is overall run by other tools (xCat, a bit of Proxmox, ISC DHCP, a custom IPAM system, etc). The way we keep it separated is via VLANs, which are only propagated to the switches we need them on. In fact, the two rackd machines we have manage machines in separate sets of racks with separate VLANs and as such they don’t have much to do with each other except of course for their association with the single regiond server.
Ours is not (currently?) a transition plan for total takeover, but in theory I guess it could be. I don’t know if any of that helps, but I’m happy to talk about our setup some more if you like.