In the earlier days of diesel era, railroads tended to run engines in fairly pure sets. This also eliminated issues of air brake schedule, dynamic brake control or mu incompatibility. Most switchers and even some branchline units were not equipped with mu, so those and mu incompatible units were dead in train when moved with other road units. If the entire consist was GP40s, then the engineer would manually be reducing the throttle if at high horsepower per axle and low speeds there wasn't enough adhesion to keep the wheels from slipping.

The power matching systems offered by EMD and GE (so presumably on Alco too) worked very simply. At 10 mph they restricted the excitation of the main generator/traction alternator to about 500 hp per axle, and slowly increased output to somewhere around 15 mph they were at full horsepower. At 10 mph, a GP40 in run 8 is limited electrically to only producing 2000 hp.

So if you had a GP38(-2) and a GP40(-2) in your consist - at 10 mph both would be producing 2,000 ho. Above 15 mph, the 38 would still be at 2,000 hp but the 40 would be at its full 3,000 ho output.

If you had a GP38(-2) and an SD40(-2) together, both are rated at 500 hp per axle, so they would have full horsepower available at 10 mph. If it was a SD40 and GP40, again at 10 mph the GP40 would be limited to 2,000 hp.

Another factor is the short time ratings or minimum continuous speed. Depending on the design, traction motors, gear ratios and horsepower, the weakest link (lowest rating) applies to the entire consist. Sometimes this isn't that big of a difference, other times it makes a huge difference.

So if you mix 4 and 6 axle units in the same consist, they will usually work together. If they have the same approximate horsepower per axle and other systems, they will generally play well together. However, if the units vary greatly from each other in horsepower per axle, minimum continuous speed rating or adhesion systems, they may not play well together or be economically practical.