Red Alert 1 was originally suppose to take place in the 70s with 1940s to 1980s tech mixed in according to a archived Q&A session nearly a year before RA1 came out. That would explain the high tech leftovers like the Apache, Hind, T-72, and M1 Abrams which some were still prototype stage at that point but it's not unfitting for the Allies to have the early XM1/M1 unlike GDI that has the M1A1. I made a post linking the source to it to.
https://w3dhub.com/forum/topic/419113-red-alert-1s-time-line/#comment-694908
You got some ideas rolling around but do you plan to add a "pure" mode to your Red Alert mod? In my preference I would just like the original units.