I'd suggest using locomotives from only one manufacturer, as it would cut down on the parts inventory that your road would need to have on-hand, and also simplify maintenance, as the shop forces would need to be familiar with only one brand.It would also make sense to standardise on the models to choose: one type for the mainline trains and something else that's suitable for switching, perhaps.
While from a modelling perspective, it might be less interesting visually than having locos from different manufacturers, I'd think it to be more prototypical.
If you want to include variety in your fleet of similar locomotives, buy the same models, but in different roadnames and paint schemes, then simply "patch" them with your freelanced road's reporting marks and numbering system.
Wayne
So your present day railroad only uses 30 year old locomotives? Um ok. Then you will need several SD40-2 locomotives. They were built until 1989. Also GP50 or GP60 locomotives. You might also want some SD40T-2 tunnel motors. Of course you don't have to paint them all in your railroad's livery. You can do what the real railroads do when they buy used locomotives and just renumber them. If your railroad can't afford new locomotives then it probably can't afford paint either.
An alternate use of the F7A, since F-units were most all retired from general freight service a long time ago, would be to power an excursion or dinner train. One F-unit with 3-4 dining cars or coaches might be a nice diversion from the usual coal trains. It wouldn't matter if the cars were all from different railroads either, as historical societies and railfan rail organizations often 'mix and match' equipment.
For a coal road still running an F7A after 1990, I expect you'd need another A and two B's. It's tradition. And the F's would have been lovingly maintained to last that long, and/or highly modified. Sounds like fun.
BN was famous for fielding ABBA helper sets over Marias Pass. And there's a photo of BN F's in iron ore service in 1980. You might track down photos of them in service for inspiration.
Staying with the coal road concept, one could add more modern power. That would be either EMD or GE. I would seriously consider getting SD38's and/or SD38-2's (See Athearn and Kato).
That takes care of the coal.
The B23-7 and C-420 will do well for "other" freight, especially MU'ed together as a way-freight local set.
For faster (than coal) through freights, you can't beat SD40-2's. They are kind of bland (except for the porches), so you could argue for something "different". But the SD40-2's would likely be THE best choice. OR. You might get something like SD45-2's. Or even argue for F45's.
Ed
Wow, I totally misread the question ...
pre-1990s era locomotives, I'd say you'd probably do well to get some dash-7s or dash-8s. Depends on how far back you want to go though in terms of age. I mean, a couple GP9s might not be untoward - but those were last produced in the mid 1960s.
-Dan
Builder of Bowser steam! Railimages Site
On my layout based in present day Northeastern America, one of the roads on the layout will be the CHR(Chicago Heavy Railroad) which runs from the Appalachian Plateau in Virginia to Chicago carrying coal. All the locomotives in the fleet will all be diesel locomotives built before 1990. I already have a SF B23-7, UP F7A, and a MR C-420 that I will paint in custom CHR liverie (will show pics when done), what other locomotives should I use for the CHR?