You will want to use a 12" toilet on the new bath. There will still be some space between the tank and wall. The other toilet problem is not quite as simple. It seems that there is no exact industry standard on rough-in sizing. For example, some toilets listed as 12" actually only take about 11-1/2". So, it may depend on what toilet listed as a 10" you try to fit on your flange. My guess is that if you do find one that will fit, it will be a tight fit. Compounding your problem is the fact that there are not many 10" toilets to choose from. If you don't get a more precise answer, you might send Terry a PM. He should be able to give you the data you need.