Quick update:
1. Tweeter fits fine (you remove the grill/etc. from the Focus tweeter, just the driver itself)
2. I looked at just putting the new mid where the original one was as mentioned before. A couple issues.
a. If you just put it in the hole, the magnet rubs on part of the window mechanism (this thing):
It only seems to add pressure when the window is down, but I didn't like the pressure on plastic parts.
b. If just screwed in the hole, it isn't pointing in the right direction, and isn't sealed to the hole in the door panel, so the sound fills the door. Overall, didn't sound great.
So, I just bought some speaker adapters to fix both problems and fit like the stock speakers:
VAUXHALL CT25VX05. Vauxhall Corsa: 2005 onwards. SPEAKER ADAPTOR FOR. Rear Door Speaker Adaptor Kit. They are easy to fit and will eliminate any rattles as well as ensuring that they fit securely. Reverse Sensor Interface.
www.ebay.com
They seem to be the correct ones, I'll know in a couple weeks when they arrive. I put the stock speakers back in until this arrives.
FYI, This is how I connected the PC2-805 connectors to the Focus wiring:
This fits the car's mid connector fine (I did put a zip tie across the connectors to keep them fitted well):
ebay.com/itm/Speaker-Adaptor-Cable-For-Vauxhall-Renault-Volkswagen-Nissan-Audi-PC2-805-Pairs/333591079298?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
I'll update once the speaker housing adapters arrive...