The RTS plugin is already in Construct.
Create a sprite, look at the stuff on the left side of Construct, and you will find the RTS plugin under "Movement".
It's quite nice. I'm using it in my game, but not as selectable tanks and soldiers but as a pathfinding AI for my enemies in the RPG-like game I'm making. <img src="{SMILIES_PATH}/icon_smile.gif" alt=":)" title="Smile" />
As far as I know there doesn't exist much 3rd party plugins yet. But Construct is still in Beta and people is probably busy with learning the app first and find the bugs in it. <img src="{SMILIES_PATH}/icon_smile.gif" alt=":)" title="Smile" />
I guess the Wiki will be the place to visit once there is some plugins available.