Aha!
I looked through the Tactics mod list last night; but I must have been too tired to see it.
And yes, many of the underground areas have different monsters depending on the party level. I remember one game where I left the Unseeing Eye quest til chapter six ... wow. Shield of Balduran was sort of necessary there.
I just finished of MR. ToB from BG1; and now to download and begin.
Armen; thanks for the link!