2 points to address here.

1. Control files are *not* in sys tablespace; they are in the location specified in your init.ora file with control_files parameter.

2. It's quite easy to determine when your tablespace is about to fill up. And if a user tries to exceed the datafile (assuming no autoextend since these are raw devices), no real harm is done other than a failure to extend a segment. However, controlfiles grow, and the size of the control file is not stored anywhere in the database (that I know of). So you can't tell how long the controlfile is. If the controlfile grows past the end of the device, very bad things happen to DBAs (usually requiring a restore & recovery).

On MetaLink, see Note:73584.1
Subject: How to determine exact size of a controlfile on a raw device

As far as mixing raw & UFS I think you can, altho some Parallel server configurations might limit your choices.

hope that clears some things up.

d.