You'll either need to place a L3 device in front of the MX which can handle this type of setup, or ask Comcast to provide the /29 as the handoff and skip the /30. This is a limitation of the MX line. Our solution in the past was to put an MS320 in front of the MX and create two L3 interfaces on it - one with the /30 for the Comcast handoff, the second with the IP space Comcast was routing to us.
Yep, that should work since the MS225's support 16 static routes. Just make sure you create the /30 interface first so the default route is set properly.
Also keep in mind you'll still need to get the MS225 Internet access through a non-routed interface. So just plug it into your internal network in your usual management VLAN in addition to the Comcast interface and the MX interface.
Yes, you can have other devices on the switch as long as they’re in another VLAN.
Create a L3 interface with the /30 from Comcast and assign a unique VLAN to it. Put a port in access mode in that VLAN and plug the Ciena into it.
Create a second L3 interface with the /29 from Comcast and use a different unique VLAN. Put another port in access mode in this VLAN and plug the MX Internet 1 port into this port. Assign the MX an IP from the /29 and there you go. You can then use the remaining IP’s in the /29 (minus the MS225’s IP and MX’s IP) for 1:1 NAT rules.
As long as you create the /30 interface on the switch first, it will correctly use that as the default route for all other L3 interfaces you create. You should still set the MX's gateway as the /29 block IP you assign to the MS225 though.