i'm a Johnson product type of guy. Humminbird, Minnkota, Lakemaster, eureka tents, etc.
I have a Helix 9 Gen4 MDI on teh bow, Terrova 24v with MDI and I-Pilot Link with blue tooth Heading Sensor, M360, and just added Mega Live. Counsel I have a Helix 9 Gen 4 MSI. and I connected everything thru a 5 port ethernet hub.
I recommend staying with the one boat network. the system works fantastic. I can control teh TM from either sonar unit, from the remote, or from teh foot pedal. The sonar units can see all how 4 transducers, MDI built into the TM, M360, Mega Live, and MSI on the transom.
As for M360 or MLive, up to you. Each has a place and a purpose. Depends on how you want to use them or how you fish. I use M360 to find cover/structure around the boat and the direction/distance from me. I am starting to use the MLive to watch my bait and the fish reaction.

Honestly, when I go fishing i start out using MSI and map an area using AutoChart. While mapping a new area I look for cover/structure and add waypoints. I also watch for fish schools. When it's time to fish I head back to one of those waypoints/schools and drop my M360 to get a better view of the area and see if I missed anything, cover/structure/fish. Then I drop MLive to fish. The MLive is new to me, so I'm still learning how to use it.
When I spider rig, I use the M360 and watch for cover/structure/schools and work my way toward them. Not sure how MLive will fit into this scene, but I'll give it a shot. That is how i use my electronics.
I also use my Bimini top in inclement weather and sit under it while spider rigging. I use my counsel unit to control the TM and poke out when i get a bite. This winter I want to set the boat up for pulling cranks, spider rigging off the back.

As for your situation, How do you want to use it? I'd stick with HB products for connectivity, one-boat-network.