I think we can use more. As long as you set the value to zero prior to your script. I have not hacked Ruby much so I can't be sure. I'm guessing we can, tho.

Guess what? I found out some stuff. At the script that prevents the player from going into the sandstorm without Go-Goggles, the Var used there is 0. Really. 0. And I think the 6000+ aren't used yet. Of course, it wouldn't be recommended to use more than 7500, but 1500 vars (in hex, nonetheless) would be enough, I suppose.

6000+ have something written to it. My friend tested 6001 and 6050, and they had to be set to 0 before doing anything.
Are these variables really unused? Maybe they're just unimportant to the game.

Some vars may seem to be used, but most likely aren't probably from thing not fully implemented into the game.

You'll just have to test them, and see.

Really, aren't 0x50 vars enough anyway? Just be careful with your usage of variables, and reuse as many as you can and you'll have no such problems.

It's possible to extend the number of safe vars/flags on FireRed and Emerald, but NOT on Ruby, as far as I know, although that's a completely different question altogether.