I just read a post:
Question was like can I ask the Immigration Officer to stamp me inside Thailand with a Visa Exempt Entry in stead of the new Single Entry Tourist Visa I have in my passport? I like to use this visa a little later.
Two times the answer was NO, you can not.
And the question was removed before I could answer.
YES, you are allowed to ASK the Immigration Officer to do this. If he decide to stamp the 30 days or use the Tourist visa is his decision, but if you do not ask you know for sure the tourist visa will be used.