My question is can I use the auto assigned MAC address from VMware or does it change everytime it reboots?
You can use the autocreated MAC address. It may only change if you manually unregister a VM from a host, and register it to another one. In this case you will be asked whether you moved, or copied the VM. In case you answer with "I copied it", the VM will get a new MAC address to avoid duplicates.
Would it be better to just go in and assign a custom MAC?
That's usually not necessary, but it may be a good idea if you use e.g. software which is licensed by the VM's MAC address.
Can I just copy the existing Auto MAC and apply it as a custom MAC?
No, at least not without manual modifications in the VMs settings. VMware uses different ranges for manually, and auto-assigned MAC addresses.
André