Portraits: I use:
medium portrait: BG:EE - 169x266; BG/BG2 - 110x170
small portrait: BG:EE - 54x84; BG/BG2 - 38x60
epilogue portrait remains 210x330 for both classic and EE
I made my EE portraits by resizing the epoligue portrait, I did not change any color encoding (but maybe someone with better knowledge should anser for that, too).
For the .BAMs: All BG1 description bams are present in the BG:EE as.bmp, which the EE games use, and the names are the same for BG:EE and BG1. Inventory bams are .bams in EE also. For Tutu and BGT, the bams are not necessarily the same as for BG1, this depends whether the bam is present or the name already used for BGII.
So, yes, a custom item using BG1 bams would call the correct pictures in an EE game, but for BGT and TUTU, it's better to check.