Sounds good! When you go to collect your two Stainzes from Glendale (or give them a ring in advance), ask Brian to change their loco addresses for you so that you won't have clashes with your two exisiting locos on addresses 02 and 03..... I don't know what kind of decoder programming capabilities your old Bachmann control system has, and in any case you're not yet used to changing these things, so much better to let someone do it for you where they have all the knowledge and the equipment to do it very quickly and easily!
As you've only got the capability to run DCC loco addresses 01 to 09 (I think that 10 is the "faux analogue" setting, isn't it?), I'd suggest probably putting the new Stainzes on 01 and 04, and put a little sticker on each one to remind you which loco address it is set to.
Oh, and I agree with Playmo, it's a slippery slope downhill from here - you'll be running out of loco addresses in no time.....
Jon.