Of course coupler height and correct flange dimensions are important. But there is another important consideration. The trucks at either end need to be adjusted differently. On one end you want the truck to rotate circularly, BUT you don't want it to allow the car to rocker side to side. On the other end you want to allow the truck more latitude to allow rotating and rocker. Its sort of a 3-point stance.
And finally, great layout work...you really move right along

And I wouldn't be too worried about a LOT of action in a small layout (dense), particularly on your first layout. I did a similar thing with my
Central Midland layout HERE.
Later you might want larger radius than 22", especially if you ever take a liking for steam.

.....much more exciting than those 'boxes with diesels'