Hi Peter!
Question 1: Is it ok, developing a free Software, thadt can "ALSO" run on Windows or Mac?
Of course yes. Project submitted to Savannah or evaluated as software for GNU must run on free (as in freedom) operating system but can also support non-free operating systems.
Question 2: Is it ok, to let Users of the own developed, free Software to do what ever the want (even if the self developing thidparty, closed source Extensions)?
Well, it depends on license that you are using for your program and on some technical details. If we're talking about GPL, you can find answer here: http://www.gnu.org/licenses/gpl-faq.en.html#GPLAndPlugins
Question 3: Whats about a License like BSD (thadts my Favorite License theese Days) wich provide freedom to any aspect of usage without any force in any direction?
BSD-like licenses are free (again, as in freedom) and GPL-compatible (excluding original, 4-clause BSD license) thus they are acceptable. The problem with these licenses is that they aren't copyleft. You can read about copyleft here: http://www.gnu.org/copyleft/copyleft.en.html
Regards, Tomasz
