>>decimals in cell labels:
This can be done in the Graph Builder version of a mosiac plot.
>>font size in cell labels
a "kinda ok" way is to use CtrlShift+ and CtrlShift-, but this changes all the fonts everwhere. Might not be what you're looking for.
>>label even the tiny cells
I believe the Graph Builder version labels all the cells (it definitely labels cells that the Fit Y by X platform skips)
>>change text color
I don't have any ideas on this. Perhaps others will chime in.