HoTT in Glorantha

For Gloranthan HOTT (a project that flickers on my radar!), I'd be tempted to go with 28mm for the sheer range of available figures and the chance to paint them up with appropriate vividness. It's easier to paint runes on shields and skin in 28mm than in 15mm!

There are loads of plastic ancients that will work well, and those give you the opportunity to kitbash and convert. Here's another of my LAF threads with some converted Uz and Dragonewts underway, for example. You can order plastic ancients by the sprue from eBay, so it's easy to build an army a few elements at a time without any major outlay. 

If you're not into converting, you could get plenty of 'close enough' options. For example, Otherworld and Fenris (Dark Fable) do pig-faced orcs that could work as dark trolls. Or GW orcs/savage orcs could be painted up appropriately, snouts be damned! The GW Gor and Ungor beastmen make perfect broos, and you can add more variety and chaoticness by mixing limbs and heads between the sets.

There are lots of available orcish boar riders around that would make decent Tusk Riders - unsurprisingly, as Citadel orc boar riders seem to have been inspired by the Tusk Riders in the first place. 

The trickiest thing is probably finding mounts for Praxian nomads. That might be easier in 15mm, as Battle Valor (for instance) make bison cavalry (albeit with brooish riders). But you can just paint horses as zebras for Pavis types, and you could probably get away with adding horns, etc., to certain horses to make sable riders.

I wouldn't bother too much with trying to match the recent RuneQuest illustrations. The older 'look' of Glorantha was a lot more achievable (e.g. chainmail was still acceptable, so lots of Viking plastics from the likes of Victrix and Gripping Beast would work as Orlanthi). I would use the new stuff for painting inspiration, though.

Also, have a look here and (especially) here.