I'd recommend you continue to use SCOM Management Servers as your Veeam Collectors. That's a good architecture model - the SCOM Management Server scales better and is more stable than a standard Agent when performing this agentless data processing.
Also, I don't think you need another Management Server. You do need to take care not to overload the RMS (or RMS Emulator) - but with 600 VMs spread over two servers, you are well within the scalability limits for v6 MP.
Do note that if you take one of the Management Servers offline, all the VMware monitoring will fail-over to one server. That's still within limits, but I'd probably recommend a 3rd server if you grow the VMware environment in future. Watch the Workflow Count view in our MP to see the real-world load on your Management Servers - we also have a monitor on that - our recommended maximum is 75,000 workflows per server. You can see this counter in view Veeam for VMware / Veeam Collectors / Performance Views / Workflow count.
And you can keep the 5.7 MP in place, if you wish, although unless you have some specific reporting needs I would recommend you remove it once you have installed v6.0. It can be a bit confusing when you are trying to, for example, create your own Groups or Overrides, and you have two sets of 'Veeam VMware' objects available in SCOM for both 5.7 and 6.0. Anyway the v5.x objects will stop generating alerts automatically, as soon as you've upgraded your Collectors to v6, because then you will only have v6-type data coming in.
Hope that answers your questions! Good luck with the upgrade, any issues please let us know.
Vice President, Product Management