To the best of my knowledge, there are no "rules against" things in clean rooms.
There may be rules against things for other reasons, but not simply because it is in a clean room. These other "rules against" come from preferences of personnel or customer requirements.
The only real "must have" in a clean room is 'particulate-free' air as measured.
How you get the "particulate free" air, be it gowning, allowable actions, processes enclosed, etc. are basically unlimited as long as the particulate level in the air is measured and within an acceptable threshold.
By way of "is it a good idea?"...I think at the time the safety shower actually gets used, the clean room air particulate level may not be the foremost concern.
If a safety shower were warranted in the area, I would not let the clean room requirements stand in the way. Class 10,000 is a fairly loose cleanroom level to begin with...protect your people.