In the early years, most roads tended to run similar units together, but that changed gradually. As mentioned, Baldwins generaly used an MU system that was incompatible with others. I understand FM offered MU options so that some FM units might have been compatible with Baldwins, but can't confirm that. Lima-Hamilton was merged into Baldwin, and I don't know what kind of MU systems they used. In many cases, early diesels were re-engined. Some NYC engines were re-engined with EMD prime movers with little or no external evidence of the change. If you saw an NYC EMD MU'd with an FM, it might actually be two EMD's with one simply wearing the external appearance of an FM. In my experience, I seem to recall that NYC adopted the mix-n-match practice fairly early, and other roads took a bit longer to do that.
The Erie almost always ran pure matched sets. Once the merger took place, Erie Lackawanna gradually started mixing things up, and by the end of that road's existence, many loco consists looked like the proverbial dog's breakfast.
On the B&O, Baldwins ran with Baldwins, Alcos ran with Alcos, and EMD's ran with EMD's until at least the early 1960's. Occasionally a set of hood units would be spliced with a road switcher serving as an ersatz B unit, but the manufacturers were generally the same.
Occasionally you might see E6's or E7's running with E8's on many roads, but they were all EMD models so that shouldn't be a big surprise. I believe I've seen photos of E units and PA's running together on the Wabash or MoPac. It seems that mismatched sets weren't very common on most roads until about the mid 1960's.
I realize the original question had to do with B units, but this discussion has branched off into a general discussion of MU practice. It might be worthwhile to add that many early units did not have MU connections on both ends. This is particularly true of cab units. Some switchers had no MU connections at all. Baldwin Sharks did not have nose MU connections, and I don't know of any that were later modified with that feature (although I do know that many Alcos and EMD's were so modified). I remember seeing a PRR ore train hauled by two back-to-back pairs of Baldwin RF16's. Since there was no nose MU capability, the train required two engine crews, so it was a true diesel double header.