How thick can I put a layer of thinset under tile ? There are a couple of places where the original tile was removed and a place under a cabinet where there was none. This area is about 1/4-3/8" lower than the rest of the floor (which is covered with tile). Im planning to lay the new tile over the existing tile.
Should I use thinset or somthing else to level the floor before installing new tile ?
Any portland cement based filler product will work for your purposes. This includes thinset. Keep in mind however that thinset can have a very high 'shrink-rate' and if you try to set the tiles with the same application of thinset filler the shrink may well take the tile with it resulting in a rippling effect and uneven tiles.
I would fill the low spots and allow the fill to dry hard before attempting to apply the tile. You may also consider one of the fast drying patching products as long as it is portland cement based and not gypsum based.