Errr... Brian, great sarcasm, but flawed.
What he didn't really mention was the ability to consist the engines together through multiple unit cables. MU cables were generally universal, and allowed a ostler to create consistes of almost anything he wanted. So in that term, yes, you could hook an E8 to a PA-1.
However...
Locomotives from differing manufacturers did not always "play well" together. This was due to two factors: load time and gearing. Regarding load time, EMDs are notoriously slow to respond to throttle changes, while Alcos and GEs have tradtionally been rapid to respond. This response time is what is called "load time". Mixing locomotives with incompatible load times would be similar to trying to hook up a Tyco GP20 to an Athearn Genesis SD70MAC -- one would always be dragging or pushing the other around. Too much of that and a coupler can break or even shear right off it's drawbar, not to mention it's just not fuel efficient.
On compatibility and gearing, locomotives could be ordered with differing "gear ratios". For example, a locomotive might be ordered with high gearing for passenger service, or with low gearing for drag freights or switching, or a mid-level gearing to be a general road unit.
In manifest freight service, it was not uncommon to see engines from multiple manufacturers mixed after the early 1950s. On the other hand, passenger trains and high-priority freights (such as TOFC, intermodal, reefer expresses, etc...) would usually have locomotives of matching manufacturer, if not model, in order to ensure optimum performance. Although I've seen vast mixtures of locomotive models in passenger consists -- E7s mated to GP9s for example -- I don't think I've ever seen a mixture of manufacturers, though I'm sure someone obscure did it somewhere.