Should work but a few comments.   1. What do you gain by stacking the transit switches? 2.  If the transit switches are Meraki then you probably need to run separate LAN connections back to them from the firewalls or another downstream switch for management.  You could assign them public IPs but that's really not a good idea. ... View more

I would connect each MX to two switches in each stack so you can have the WAN connection survive any single switch failure in the stack.  If you're using 10G connections then any more than 2 connection per stack isn't really gaining you anything beyond protection from the unlikely case of both up-linked switches failing at the same time and spending huge amounts of extra money.  Put them on separate AC and UPS  circuits and this becomes an almost impossible occurrence.   If you're up-linking with 1G connections then you're never going to be able to take full advantage of the throughput of this MX model. ... View more

Just keep in mind that while you can create two VLANs and assign 3 ports to each making this work, by doing so you are leaving the MS-220-8 as a single point of failure which somewhat defeats the purpose of using a second MX, dual ISPs, etc.  Might be better to use the MS-220 for one ISP and use the other ports as edge ports then grab 3 ports on another switch for the other ISP.  ... View more