As for the mounting of riders, the final version will be peg and hole, and the riders are completely swappable. It is pretty easy to add magnets if you like (I can't do it in the package as the costs are pretty high), but my prototype uses magnets for exactly that reason.
As for scale, I started at 15mm, but the riders just came out so very small. Going 28mm would have made the dragons enormous, and unwieldy for play. We went to 20mm so that the riders, which are a big focus of the game, can be seen and painted in detail. Honestly I think they'd mix well with 18mm figs, and the dragons, alone, could mix well regardless since they are big at either scale.
As for ground troops, one of the things I really wanted to do was allow people enough options so that they can play the way they want to. I didn't want a game that is just competitive tournament focused, though you certainly can play it that way.
I'll go into it a little more in a future game play article, but at the basic level, there are two primary ways you can come up against ground units.
The first is in a single mission (or mission within a campaign) where you are trying to take out ground units, or prevent ground units from transiting across the map. Some ground units can be fixed- like an archer tower, or a target, like a castle that has to be hit enough until it is destroyed. These are co-operative play focused with ground units working in a pre-programmed and reactionary fashion.
The trick in these missions is to plan your strikes so you do the most damage while minimizing your exposure to ground fire. I'd say these missions tend to play a bit like the bomber missions or balloon busting missions in Wings of War/Glory.
The second mode where you would face ground troops is in what is called Conquest Mode, which is competitive focused. Both sides get a force of ground troops as well as dragons. The goal is for one force of ground troops to destroy the other. The side that does that first, wins. In these battles, your own side has to defend your own ground forces, assault the enemy ground forces, and try not to get hit by opposing dragons all at once. It's a bit more chaotic.
In my game ground units are formations- a formation of archers, a formation of cavalry, a formation of men at arms. They are on thick cut out flats because I want the focus on dragons (on the one hand) and because you will be flying right over these units (on the other hand). They aren't individual heroes, or individual figures, but masses of troops, so they can absorb damage, and if you get too close, dish it out to you. There are attached anti-dragon siege engines that you can bring along, but they move very slowly.
There is a third form of ground unit, and that's the Goliath. He's a special case in that he's a 100 foot tall crazed giant looking to eat your dragon (or anything else he can get his hands on). He's tall enough to engage dragons in melee while they are in the air (unless you opt to soar, which is the highest altitude, but you won't be able to interact with him, and he won't be able to interact with you).
If the game does well, our next expansion will include a sea supplement with dragons over the water against sailing ships. One of the factions, The Pirate Lords of the Blood Sea, is pirate themed, so we're already having a lot of fun with that.
