Can you put files in /tmp? A friend of mine was telling me never to do that, it could bring down a machine. Is this true?
Depends. If it's actually a temporary file that's fine.
There are two situations where it might bring a machine down (or at least impair operation):
There's no limit on it and you fill the root disk. This is even more of a problem if /var is the same slice/partition.
It's RAM and there's no limit so you use up all of the RAM on the machine.
But in general, the point is as a place for users to store temporary files (i.e. ones that may be deleted by the administrator or will disappear if the machine reboots).
It's what it's there for. The only potential problem is if you run out of space. You should be aware that the contents are generally not preserved across reboots.
If you're writing software for a multi-user system, make sure you create temp files securely. If writing shell scripts, the mktemp utility does this easily.