Should. It really isn't much of a mod, it just blocks them from showing up in the starter list and then also a a character that can show up through random events (including events from other mods, that I HAVE tested in my games).
I haven't tried what you're asking but my guess is it works, the vanilla already in the game remains (nothing is telling the game to delete them) but future characters just don't appear. Either way, just try it out, it won't break anything.

Will this Mod work in a Game already in progress? Specifically the Trade Scenario; will it prevent the Vanilla Survivors from Applying for Jobs?

Also... My Current game is using 3 Modded Survivors; Just out of Curiosity (IF, of course, this Mod Worked with Current games), what would happen if a Vanilla was in your game when you initiated this Mod?

Nice

Murder a few extra bugs with Corporal Hicks and Johnny Rico!

I can't claim the idea was mine, though. Another guy did a mod similar to this that gave me the idea but his had a problem in that if another mod altered the vanilla characters it prevented his mod from actually getting rid of them. Asked him if he wanted this to upload instead but he said he was pretty much done with the game and said I should throw it up. As far as I've seen; this bypasses that flaw and full on prevents them from showing up under any circumstance.
The mod that was screwing with his for me was Beyond Stranded. I like the extra events it adds (and excavating, when it works!) and was super frustrated when Maki and Ken and the gang were showing up when I wanted all my pop culture characters instead.

But I digress; hopefully you and others find that it works as well for you as it seems to be working for me.

But what will I do if Maki doesn't sleep half the day away, get up, eat a meal, play a musical instrument, and then go right back to bed?!?!

Great mod idea bud. Will try on my next go-around.