Mongo Rebellion
1 Aerial Hero General (Flash Gordon on rocket sled, possibly with Dale Arden)
1 Airboat (Captured rocket ship)
1 Cleric (Dr. Hans Zarkov with various devices that negate Ming's
technology. Dale Arden may accompany him)
2 Fliers (Hawkmen)
2 Warband (Lion Men)
1 Shooter (Arboreans with bows and guns)
2 Lurkers (Arborean ambushers)
Option: Sneaker (Dale Arden)
These lists are based heavily on the 1980 film. but with odd bit creeping in from my dim memories of seeing the original 1930s Republic serial when it was shown on TV in the 1970s. There's even a hint of the 'Star Trek: Voyager' episode 'Bride of Chaotica'. I'm sure there are many other races and things that could be included, but I suspect that they will just be variants of the above, and that what I have presented here covers the basics, and has the right flavour.
Figures? In 15mm, Flash could be done using the one man fliers from the Peter Pig humanoid Martians. Irregular do a bald sorceror which would do for Ming. Hawkmen are made by MJ Figures. Lionmen could be made from hairy barbarians with tails attached. Arboreans all look like Robin Hood in the film, but suitable figures should not be hard to find. Ming's soldiers could be converted from various 19th century ranges; 1870 Prussians would give the right feel, rearmed with halberds, or with rifles converted to rayguns. Packs and other equipment could be covered by cloaks. The Mongo nasties can be cobbled together from any assorted monsters and creatures in your odd figure box. I'm not sure about the rocket ships, although the Federation Starships from Reverisco have been suggested.