Aye, spnpccharacters.xml is very easy to edit. Get yourself Notepad++ and it'll give you a nice layout for working on it. As MetalJacket said, make sure you backup the original first.
What I do is save the original somewhere, then edit the file in the game folder. When I've made a couple of changes I load up the game to check I didn't mess up - if my game loads it's all good. If I did a bad copy/paste it'll crash on loading the save. If it loads fine with my new edits I then save a copy of the edited spnpccharacters.xml in a separate folder before editing the main file further. That means if I've edited 30 units and then make a mistake that causes crashes I can just revert to my previous edit/update without worry and resume.
The other thing is that when hotfixes come, if the devs make a change to a unit in spnpccharacters.xml it will override the file in that folder so you need a copy of your edited file somewhere to drop back in.
Anyhow, I've been editing units for the same purpose as your op. You'll often see units with 3-4 equipment sets - some are mostly identical just with weapons switched, others have different armour pieces too. You can change it however you like. Personally I'm working towards most lower tiers having a bit of variance to reflect recruits being given what's available and higher tiers being more uniform, plus I'm giving some factions a bit more focus. Empire I'm going down a much more uniform path even for recruits, while for Battania I'm keeping more variance throughout to reflect more of an individual approach to gearing.