You can mix and match E8 and E9 models from Lionel, MTH, etc., with no more than the usual caveats (different runs and different model makers have slightly different shades of the paint colors, etc. and put different levels of detail into their models, so sometimes the models won't match although the real locos were nearly inditinguishable).
The loco set below is the best example of this I have ever seen. The two A units are Lionel Legacy E8s, the B unit is an MTH E9 dummy. The major differences you see are that the MTH model has no snow guards on the roof top intakes, and that MTH and Lionel use slightly difference metal for the grills. Paint is a different shade although so close it is not something you usually notice. Graphics are essentially identical. The trucks are amazingly similar, too - nearly identical in details and paint. Amazingly, this MTH E9 unit even came with a correct road number (949b) to match my Lionel units (949 and 950).