I'll do my best to help based on my limited knowledge of vehicles.
-PNG shouldnt be a problem, pretty much the same thing as a .TGA.
-There is a scale factor that you need to follow. I know this because i scaled my AT-TE way too big on first attempts and it didnt work right in the game (i cant remember in modview).
If you load the JA skeleton, you'll have the correct size and units for a player model. When i set the default units within max to "international", the player's head stops at 104 units, the maximum scale for a vehicle is roughly 275-280 units.
-If i remember correctly, there is a limit on the number of frames each animation sequence can have, the magic number is 100 (or 99 + 0) i believe. That would be the cause of "invalid enum" error.
-I believe the scale in the .NPC file is a conversion for in-game units. You can see the units of the box in modview, its mostly used to create the bounding box of the object, for collisions and whatnot.