Thanks to all for sharing your thoughts. It is really appreciated.
I will try to express what are the results for me and ask yous to correct me when I am wrong. I guess we will write some sort of manifest for our project to define our projects policy and the way we interpret and work with the GPL.
* Anyone is allowed to write modules/add-ons for our project and distribute them for any fee they would like to charge. They may decide to only send the sources when the fee has been payed. We as the core developers of this project can only kindly ask anyone to send us the sources when they want them to be reviewed or integrated in our project.
* If these add-ons/modules are integrated in our work the whole work is under the GPL with all consequences.
* Noone can limit the rights of distribution when the customer has received the sources. Anyone that has paid any company a fee for their add-ons/modules is entitled to distribute this derived work at any rate they want. When the customer decides to share the sources as a contribution on our projects site the company that developed the add-on(module cannot do anything against this.
* We can define that a company that is not willing to share the sources with the community in the way we do it (for free, immediate release) is not allowed to announce his product in our forums as we have clearly written that that will be seen as commercial advertising which is not allowed in the forums.
I hope I have learned my lesson well :-)
Thanks for your support.
Jan Wildeboer