Currently, the topology loader forbids core ASes from having peering links (i.e. links with "link_to": "peer"). The validation check is here: https://github.com/scionproto/scion/blob/4a8bc98ec7d969dd622c5a90fe677e3c980aaa88/private/topology/topology.go#L544-L550.
As far as I'm aware, there is no technical reason why core ASes cannot have peering links.
Core ASes should be able to have peering links to any other AS, i.e. to both core or non-core ASes in both the same or different ISDs.
Currently, the topology loader forbids core ASes from having peering links (i.e. links with
"link_to": "peer"). The validation check is here: https://github.com/scionproto/scion/blob/4a8bc98ec7d969dd622c5a90fe677e3c980aaa88/private/topology/topology.go#L544-L550. As far as I'm aware, there is no technical reason why core ASes cannot have peering links. Core ASes should be able to have peering links to any other AS, i.e. to both core or non-core ASes in both the same or different ISDs.