actors (mostly on characters & likely the map), huds (character's hud only), & models (like effect & weapon models) including ui/models/mannequins are the most example to be used for PSP version.
Additionally, there has been a confirmation when Julio & i testing Hulk mesh, set in PS2 IGB format for PSP gameplay, since the 3DSMax 4 - 5.1 Alchemy script has both PC & PS2 settings to export to IGB, a custom PS2 setting mesh still won't work on PSP version.
I dunno if it may work on original PS2 itself, also dunno if the model error is likely because of not recontrast the texture size into a same size as PS2 ver. (256x256), or even after resizing the texture size is still error