Bob,
Your right, of course, and that is what I have on my HO layout - Bachmann and Athearn engines with Atlas, Athearn and some unknown makes of rolling stock and passenger cars. Only issue I have had is with one or two of the rolling stock not meeting up properly with the others couplers. Once I do get the layout operational again, I will be going though all of my engines and rolling stock and replacing any plastic wheel sets with good metal ones and all of the couplers with what ever is the best metal couplers to get. From what I understand, they will be Kadee Sets if I am not mistaken.
Anyway, that's all HO stuff - my N Scale isn't really big enough (at the moment) to worry about those sorts of things; however, one of my main reasons for buying the same make of say rolling stock, is to ensure that the couplers all match out of the box. Now I am looking at buying a few pieces of rolling stock, now the xmas box cars are back in storage till next year, err this year, I am going to need other rolling stock.
To that end, I am thinking about getting 3 or 4 cars but don't know what as yet or what makes to buy. All I know is that I don't want all box cars or all tank cars or all of the same thing. I do want a little variation of rolling stock but also want it to look natural - if that can be done with only 3 or 4 cars. I am saying 3 or 4 cars as I think anything longer on the layout would look a little funny, especially when you add the engine and a caboose.
Total track length is only about 68" long, so I am open to ideas and thoughts about the rolling stock - type, make and amount.