15mm is not just a smaller 28mm and it has taken me some work to try to find how much detail to add and such. For example, I based my minis on some 1" washers which looks totally out of place. I found some 1/4" which look a lot better, which you can probably tell from the pictures.
Cool Find: Burger King Star Trek Shuttle - I gave it a new paint job (it had some nice detail that came out well with a black ink wash) and I think it looks pretty cool as a troop transport. Shown below with some clicky Mech Warrior Shock Troopers I repainted (5 to a base for about $0.25 per mini with shipping).
With some Spriggan Miniatures SPUGS (space bugs):
15mm Mech: shown with the clicky Mech Warrior troopers is a 28mm armor from the AT 43 game that looks good as a mech for 15mm (was small for 28mm imho)
Rebel Minis Special Forces: I really like these guys, incredible amount of detail