first of all, those screens look gorgeous.
now i'd love to help you guys out as a playtester, but unfortunately for me i've really got not much time to invest, and i'm a sporadic user at the moment, meaning i only get to go online every once in a while. i can however give you a few tips, at least, the stuff i'd expect from a topnotch RTS on a handheld.
Frame rate. Lotta factors to work with here, mainly how much stuff is showing onscreen, and how much stuff you got going on (including offscreen action) inside the DS.
Graphics first. As i said, those screens look gorgeous, but if it drags the frame rate down, might wanna consider using simpler ones. A ton of action is damn fun, but slowdowns can sort of put a damper on things (of course, it can BE sorta helpful too, when in a pinch). Some slowdown is bearable, and unavoidable mostly, on the DS, but too much is too much. Look at Plants VS Zombies, everything looks great, but the action is still very fierce, and lags only appears when there's too much stuff going on. When it gets to that point, lags are unavoidable, so a good thing may be to
set a limit to how much stuff can be onscreen at any one point. Ensures you got top speed all throughout.
Also, don't forget
visibility. Your screens, maps, menu, they all look great, but if you can't make out the units from the background (objects look vivid, overpowering the unit sprites), then it'll be confusing as hell trying to micromanage.
Also,
put a unit/population cap. Find the max you can put for total population on one battle, then split that maybe fourways (if you're planning on making it up to 4P free-for-all, which you should, on local wifi too), then find the max amount you can animate onscreen. Then tweak that. Try to give an impressive amount onscreen, like maybe higher than 40. That amount of units wreaking havoc with no lags should be a ton of fun to watch. Bottomline: (reasonable) population caps should help with the framerate.
On the topic of
micromanaging, try to work out a way to be able to select individual groups of units, either through face/shoulder button (or button combinations, which will let you do more on a limited amount of buttons), or maybe small onscreen interfaces (maybe a dropdown list, icon list/grid, even radial commands, your choice). Being able to effectively control even individual units on the fly will be SO MUCH WIN. Also, maybe work in some of these commands, like
"Rally to leader" where all available combat units will converge on a specific unit you're keeping an eye on, or
"Rally to combat" where available combat units converge on the first enemy contact (like when you get attacked, your combat units converge WHERE THE ATTACK HAPPENED, especially on homebase or critical resources). These are of course, just feature suggestions, and will require some pretty smart AI scripting.
And on that topic,
AI. God, please make 'em smart as hell. Like intuitive smart, if you can.
Pathfinding! Make sure Pathfinding algorithms are good enough to be able to send a resource drone somewhere at least two or three screens away. Half the fight is won if your units are smart enough to make their way from east to west without getting clogged on a bottleneck because one unit does not know to walk around a wide cliff.
There's just so much stuff i wanna add, which makes me wanna sign up more, but i'm afraid i might be reaching this post's character limit, i dunno, so i'll leave things at that, and just keep a close watch on your project.
More power, i hope betatesting goes great, and make it so good that maybe we'll see it actually PUBLISHED.