The cabling used for the HA are standard ethernet cables.
You could successfully cluster an H to an HE device. Naturally any cards in the slots must match on both devices. But you will need to be careful that you don't try to use any features that require the HE model. If I remember correctly this basically means you will be limited in Junos upgrade version to the top version allowed on the H model.
The other caveat is that technically this configuration would be "unsupported" as Juniper does state that clustering is only supported on exact matching models. So you may have a JTAC case where the clustering itself is an issue and you will be told no further troubleshooting would be done until the devices matched in hardware. But my general experience with JTAC is that they do help until it becomes a serious possibility that the support issue is related to the "unsupported" configuration. they are not like some vendors where they do the litmus test up front and close the ticket regardless of the issue. However, there is a risk of not getting support when you need it.