Are adjustable sighted pistols OK to use in side matches?
Depends on the side match and the sanctioning body.
In SASS the most common adjustable sight pistol you find is the Ruger Blackhawk. You will be able to use it in most side matches outside of pocket pistol events. The other ajustable sight revolver you'll find is the Beretta Laramie. It's ok too.
NCOWS restricts the use of the Blackhawk as it is not a replica of any pre-1900 design. The Beretta Laramie is ok as it is a replica of the S&W New Model No.3, target model.
If you are shooting at a CAS club that is independent (not SASS, not NCOWS) they may have their own local rules.