Hopefully the 3 main switches are stackable.  Simply stack them,  RSTP won't cause any issues if the network is loop free, so I wouldn't worry about touching that.   Use a pair of cables to the fourth switch, and combine them into a channel using LACP.   Ports that go to devices should be configured as access ports.  Ports between switches should be configured as trunk ports.   I would tend to connect each MX250 to a different single switch (one MX to one MS, and the other MX to a different MS), to maintain everything loop free. ... View more

Are your multicast sources on the same subnet/VLAN as the multicast receivers? If they are on different VLAN's where does the interVLAN routing happen? On the CORE switches or Firewall?  Or do you have multiple hops between your multicast sources and receivers? If they are on the same VLAN.  Are you running PIM on the first hop or do you use IGMP querier on the CORE switch? ... View more