I would agree with that, one large procedure library will hinder the work of a group of programmers. In our experience, multiple chek-out and merging afterwards was not workable.
We had 10 programmers and because our code was not in small enough libraries, checking the merge meant much more work for each multiple change.
More smaller libraries was the answer.

And we turned OFF multiple check-out instead of leaving it on.
It makes using the VFP integration harder, because developers must undo checkout on the project after joining or updating it. But it does prevent problems with merging.