Have you tried

https://trn.trains.com/railroads/rosters/canadian-national or  

https://cptdb.ca/wiki/index.php/Canadian_National_Railway

CN Historical Association Cyclopedia

Neither of the above links have any of the information he's looking for, but the CNR Historical Asssociation has fleet histories on their web site:

http://www.cnrha.ca/CNcyclopedia

YouTube

Seriously, you can learn a lot from old videos on YouTube. When I thought that I wanted to model 1986 Arizona, I found some great videos that showed me that Arizona was the land of the Southern Pacific GP35R and the SD40T-2, with a few GP40-2's and SD45R's mixed in

YouTube

YouTube has been very helpful. 

Retirement information has been difficult to figure out. CN seems to be a unique RR in that especially in the 1990s and early 2000s through acquisitions they had a large roster of older power then most class 1s. 

I have narrowed down a few models that it appears CN would have spun off right around 1995

Trackside Guide

Is a yearly publication here in Canada.  It details more rolling stock info (primarily locos, but also vans, maintenance and OCS equipment, passenger cars, etc.)  than you'll find anywhere else for CN, CP, and other Canadian roads.

Contact me privately, Bob - I have the 1997 issue, which will likely answer your questions.

Blair Smith

RS18

Would anybody have a rough idea of retirement dates for both CN and CPs RS18s?

Good read

https://rapidotrains.com/master-class/ho-scale/mlw-rs-18-master-class

CP Rosters

http://trainweb.org/galt-stn/cproster/main.htm

http://mountainrailway.com/Diesel%20Roster%20Page.htm

CN and CP rebuild/retirements

CN's RS-18s were rebuilt and renumbered as RSC-14s in 1975, with their original 2 axle "B" trucks swapped for 3 axle "A1A" (centre idler axle) trucks and de-rated to 1400 HP. These were retired by the mid 1990s.

CP rebuilt most of their RS-18s to low nose RS-18u in the early 1980s and sold or retired those by the late 1990s.