There is an easy way to change commas to decimals and vice versa:
set dp comma
or
set dp period
It can also be changed permanently by adding the option , permanently.
In order to set a limit for the number of decimal places, one simply has to set the format. For the list command of a variable wage, one would normally type
list wage
which gives lots of decimal places, depending on the definition of the variable. In order to restrict it to (say) 2 decimal places, one would type
format wage %9.2fc
or (for using comma):
format wage %9,2fc
The result can be seen by entering: list wage which should display the correctly formatted numbers.
There are lots of other ways to format numbers. For a general overview, press help format.
To the best of my knowledge, this formats are not directly applicable to statistics like sum or regression output. Check the blog-entries for exporting this information. Many export commands support formatting of numbers (see e.g. here).