Rich,
Apologies for the slow response to this message.
Can you please clarify what you are trying to do? I just created a game with a 1970 Lahman league and a 1977 Lahman league in the same game. Is that what you are referring to? If it is, it appears to be working for me at least, so either there's something amiss or it may have been fixed in the beta version I have.
I'm not aware of any way to load all historical players easily into a single pool, but if you're interested in seeing that happen, I recommend you create a post in the Suggestions forum with details about what you'd like to see added in the future.
Let me know!
Steve