I knew about the wobbly rule but I'm not sure if it applies I'll post it for reference:
WOBBLY MODEL SYNDROME
Sometimes you may find that a particular piece of terrain makes it hard to put a model exactly where you want. If you delicately balance it in place, it is very likely to fall as soon as somebody nudges the table, leaving your painted model damaged or even broken. In cases like this, we find it is perfectly acceptable to leave the model in a safer position, as long as both players have agreed and know its ‘actual’ location. If, later on, your enemy is considering shooting the model, you will have to hold it back in the proper place so they can check if it is visible.
If that's all there is it leaves a lot to the interpretation of "wobbly". In my example I could not balance any more models in any way so I guess the rule as worded would prevent me of putting more than there is on the picture. The "it's very likely to fall" seems to indicate that there has to be a change of not falling. Which would not be the case for any other model.
In my particular game it did not matter because it was a totally friendly game but I don't like the general solution (at least with the scenery I have available) and not just because my army is the
CC horde one :-P Maybe we'll have to establish some house rule beforehand next time.
Anyway... I compiled the two other relevant rules (I guess) in case someone stumbles with this post and wants to take a look at it:
MOVING (pg. 178)
A model can be moved in any direction, to a distance, in inches, equal to or less than the Move characteristic on its datasheet. No part of the model’s base (or hull) can move further than this. It cannot be moved through other models or through terrain features such as walls, but can be moved vertically in order to climb or traverse any scenery.
RUINS (pg. 249, optional)
The galaxy is littered with the remains of once-proud cities. Unless they can FLY, VEHICLES, MONSTERS and BIKES can only end their move on the ground floor of ruins.
INFANTRY are assumed to be able to scale walls and traverse through windows, doors and portals readily.
These models can therefore move through the floors and walls of a ruin without further impediment.