I have a 6x6 ceramic tiled bathroom floor. (white,glossy finish) I would like lay a new 12x12 tile over it. I have read that laying tile over the glossy tile may not be a wise idea. is this true, even if it is sanded with a belt sander and cleaned. Then use a modified thinset to adhere? Any suggestions would be appreciated.

So long as the original floor iw well adhered and no cracks in either the tile OR the grout, you should be fine to go over it, even being glossy. No need to use a belt sander, either. Palm sander will do fine. You just want to scuff up the finish-- not sand it off!!

As for a thinset, what you want to use is an UNmodified thinset mixed with a liquid latex additive full strength, such as Laticrete's 317 thinset mixed with their 333 additive, or Mapei's kerabond thinset mixed with their Keralastic additive.

We just did this last week, and we did remove the gloss, but it was also recommended that we use a leveling compound. We did this, and it is a little messy, but the floor is entirely level.

Unless your original floor wasn't flat, I can't see the reasoning behind the SLC. It would seem like a waste of money, to me, being that the cost is alot more than thinset, so you're not even saving anything by filling in the grout joints to flush. I hope that doesn't come across as being offensive, because that's not my intention.

Thanks Bill for the suggestions. I will tackle it and let the board know the outcome.
Much apprreciated knowledge!