I had my Idolos in a test cage with a big metal lamp actually TOUCHING the top screen. They wouldn't sit UNDER the lamp, but riiiiiight next to it - maybe 3cm away, and gather around it like it was a fireplace. And it probably wasn't far from it. Easily 120 degrees at the edge.
As for crowding, I found that as long as they're kept WELL fed (big, healthy looking, almost bulging bellies), the next best thing you can do is have an uncluttered habitat. They run away, stare down, or just snap at each other, if they can see the threat coming. Leaving molting as the biggest remaining threat (they love chomping that soft, fresh flesh). So, the final judge of crowding seems to be - do they have enough space to molt in relative safety, without appearing TOO tempting to their peers.
And remember - once you have adult females, all bets are off.