Apparantly it's just a case of renaming the IWD2 BAMS to match BG2 ones you want to replace with the same number of slots.
I did some counting last night and this is what I noticed (I only recorded good ones):
IWD2 creature Files Number of slots Weapons Casting
White Abishai MABW 24 No Yes
Bugbear (light armour) MBU2 20 No No
Bugbear (unarmoured) MBUG 20 No No
Half-dragon (female, green) MDH1 26 No Yes
Half-dragon (female, red) MDH2 26 No Yes
Drider (wobbly breasts) MDRF 92 Yes Yes
Drider (male) MDRM 82 Yes Yes
Dark Tree MDTR 20 No No
Giant (smallish grey one) MGFO 22 No No
Goblin (armoured) MGO5 20 No No
Goblin Shaman MGOC 24 No Yes
Goblin Worg Rider (helmeted) MGW2 20 No No
Goblin Worg Rider (unhelmeted) MGWO 20 No No
Hook Horror MHOH 20 No No
Cambion (male, dual-sword - Isair) MISA 26 No Yes
Lemure MLEM 20 No No
Cambion (female, mace - Madae) MMAD 26 No Yes
Malarite Priest MMAL 24 No Yes
Orog Priest MNOS 24 No Yes
Orcish Shaman MOR5 24 No Yes
Half-dragon (female, blue - Sherincal) MSHE 26 No Yes
Svirfneblin (unarmed) MSV2 24 No Yes
Svirfneblin (pickaxe) MSVI 24 No Yes
White Dragon (smallish) MWDR 26 No Yes
Werebadger MWER 24 No No
Worg MWOR 20 No No
Wererat MWRA 20 No No
Yuan-ti Sorceress MYUH 24 No Yes