Greetings!
Sounds like they have assigned you a /30 and a /29 public address block and the ISP is simply routing the /29 to your CE device (MX), please clarify.
You should be able to setup the LAN side of your MX with the /29. Depending on how many ports your MX has you could plug in your addition routers that you want to have a public IP into the MX LAN ports. Just ensure the LAN side MX ports are in the same vlan (/29). If you don't have enough ports you might need a switch for extra port density.
Even if you were to do the above keep in mind all traffic from your downstream router with a public IP will NAT to that of the MX's public IP /30. Probably not ideal, unless you want to run BETA code that support "no NAT".
Because of this limitation I would use a different router at the edge where NAT isn't enforced and setup like above and connect said device to a switch to extend the /29 vlan to accomodate your additonal routers etc.
or.....you could do 1-1 NAT on the MX to your downstream routers, but the public IP won't be assigned to the physical WAN interface of the downstream router.
So a few options here, hope this helps
Cheers