VLAN Implementation - Routing & DHCP Helper

I am working on segmenting out our data traffic to individual vlans.  I would like to have as much in place and configured on the switches as possible before reassigning all the ports to the appropriate vlans.  Is it safe for me to go ahead and enable the routing on our layer 3 switch and create the dhcp listener that will forward the traffic to our dhcp server ahead of time without affecting our live network?

Currently the network is 1 single broadcast domain on vlan1.  Since all traffic is on that single vlan1, I was hoping to do all the configuration, then just open a couple ports on each switch to test traffic and make sure it all works.  Then later on assign the bulk ports and be done?

Could I also define the default route to our firewall since the traffic by default is heading that direction anyway?  Would that have any impact?

Any suggestions would be great on how I can get the most done during business hours and minimize downtime for the main cutover.

Thanks!