it was/is quite common. Just the other night I was pondering this exact subject as I was watching a DVD documentary that I have about Amtrak. There was just about every conceivable combination. There was even one part where bi-level cars were matched with some of the older Heritage-like designed cars! The one thing that I did notice was that the engine combinations were very consistant. I never saw a F40 with a 42, or a Dash 8. Hmmmmmmm......there must be some reason there. here is an example of a "mix"... http://trainweb.org/mdamtrak199/amtpp/sl2440altoona.html
This happened but pretty much only in the early 70's right after Amtrak's birth, by 75-76 this was rarer and rarer.
I ride Amtrak pretty frequently between Phila. and NYC. I never see any mixed phase trains. Maybe it's different in other parts of the country.
The NE corridor, being electrified, used primarily PRR heritage equipment in the early Amtrak days (Metroliner sets, GG1s). As AMTK obtained their own equipment (P42s, AEM-7s (aka "Swedish meatballs")), and Phase I Amfleet equipment, things were still pretty uniform. Farther to the west, there was a hodge-podge of whatever locos and passenger equipment was available. As the years went by and the AMTK roster grew (F-40s, Superliners, etc.), much of the old "heritage fleet" equipment was retired. http://www.on-track-on-line.com/amtk-roster-cars.shtml
That's because you are in the corridor. Try out here in the midwest when Amtrak only comes by 4 times a week and you have a vastly different set of circumstances.
My favorite AMTRAK site with enough photos to keep one busy: http://adsl-69-213-240-177.dsl.dytnoh.ameritech.net/amtrak/index.html
I've seen a number of shots of F40PHs with P42DCs, and of P42DCs with P32-8BWHs. Even a number of cases of F59PHIs with others, though this is unusual. The P32-8s only run in the west as far as I can tell, and of course all the F40PHs are retired.