With two micro servo each they can measure about 45x45x45mm. Would they be able to climb?
A solution for automatically landing in a maneuverable position: conic wheels
Docking could be done with magnets, so the connection will self-align to a certain degree and navigation will not need to be as accurate.
Option 1: e.g. three magnets (to avoid offset docking) on a rotating plate so that the magnetic orientation can change.
Option 2: one (strong) magnet in the center.
This would mean the robots would not be symmetric, but have a front and rear side. Thinking about it, that can be an advantage: When colliding, the part before the hinge would need an upwards impulse. This might be easier to achieve when it is passive (no actuator) and thus much lighter than the part behind the hinge.
Worth a try, I think.
Some scribbled variations on the Upend idea.
The hinge could be either integrated in the robots, or the connection between the robots. I think I prefer the former, because a hinge could make the connection weak.
Falling and landing
A symmetric construction avoids the problem of the robots landing on their back when falling. The robots could also be round (barrel-shaped) so they will – hopefully – automatically turn when landing on their sides.
Maybe the hinge could automatically block when the robot is upended, keeping it from keeling over? The robots could stick together on their flat sides. A connection in the gap (e.g. magnets or a slightly sticky surface) might also help to stabilise the tower.
One of the physical problems to solve is that the body of the robots has a shape that is easy to be climbed, also when they are upside down. In the case of the tank-robot alternative, the solution could be to add a sort of self flipping flaps, that turn with gravity.
Thinking about how the robots will seek the heap, a possibility is to set a beacon, like a light, that would serve as navigation guide for them. Another option is that they also emit light and the accumulation of robots, like a heap of them, would be the strongest source of light, acting as an attracting beam.