You can create multiple subnets on an MX, then permit the associated vlans on a trunked switchport. Add L3 firewall rules if you need to segregate subnets.
Go to Security>>Addressing and VLANs>> select use vlans check box then add your subnets. below that is the per port vlan settings for the MX and for switches, assign subnets to individual ports
Correct, it is not something that can be done with Meraki but more importantly its not something you really want to do.
I've had to do it in the past when I was a scrub because my hands were tied and the design wasn't implemented correctly, but otherwise I would advice against.