If you are doing skirmish level, I don't see any reason to go smaller than 28mm. Presumably, you'll be doing small patrols or platoon level games? Plus, as mentioned above, the Paul Hicks Gurkhas and Chindits are just a little too nice to resist! They have separate heads you know, so you can get some good variety on the figures.
I'm actually working on a Chindint list for Chain of Command (my new favorite game ) right now. I also always thought that the two hour wargames' Vietnam special forces rules would be easily adaptable World War 2. What rule set are you planning on using? That might have some impact on the scale you choose.
Ax