While the size of the pen is certainly not small, there may still be problems with the skinks if any one is overly aggressive/territorial. First of all, it's very difficult to sex them but even if we allow that you have accurately sexed individuals, some individuals are highly territorial while others are not, regardless of their sex. Even with plentiful hiding spots, you may end up with serious fighting and physical injuries. Bottom line, you wouldn't know if the group would be compatible until you put them together.
The height of the walls and the plywood should be sufficient to avoid their climbing out, provided there's nothing propped up against the inside of the walls.
I hope this helps.
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Edward
Carpe diem