The purpose of scalling is to remove the calcalus deposit that are form through the time and cant be remove by regular brushing. Increase the frequency and by using the correct brushing technique will only make the formation of calcalus much slower, but it will not halt the production of the calcalus.

In normal healthy person ( no shaking teeth, no gum bleeding/swollen ) , it is recommended to do scaling once every 6 months. For those who have shaking teeth, gum bleeding, gum swollen ( Medical term, Periodontitis ) has to do scalling every month until the condition improve, then you can increase the duration to 3 months, and then to 6 monthly.

Hope this helps 

I think all the gov clinic is the same, haha.
Errrr... Yes and no to your second question. It really depends on the clinic and the doctor. Like my clinic, we have a duty roster, if im working for today, all the patient that come today will be seen by me. So if you want Doctor A, you ask when is the day he duty and you come that day, or simply ask to put your appointment under his/her name. BUT, all clinic have their own system, try ask if you can choose your own doctor. This is common especially for woman patient who want to be treated by woman doctor.