Will I run into any issues migrating my existing VM's or Vcenter because the current hosts are all running standard vswitches? I will need to migrate these vm's to the new hosts with the distributed switches
There is a process for migrating VMs from the port group on your vSS to the port group on your vDS - best done out of hours.
How many distributed switches do you build, or whats the best practice for separating things out? I have Iscsi storage so maybe that should be on a separate dswitch?
Same practice as for vSS
The physical nics you add from a host to the dswitch are still used for communication on that host? Curious in regards to failover/disaster recovery models? We have a single vcenter that manages our data center and our DR site. They are directly connected by our own Fiber.
The basic way that pNICs are used with a vDS is the same as with a vSS - it's the "control plane" of the vSwitch that you are centralising with a vDS, not the "data plane"