Can i put ads in a secure members only area ? google wont be able to see the ads without having a username and password for my site... is this legal ?
Yes you can although it is NOT recommended. The reason being that only your members will have access to the ads so any clicks will come from the same 'few' IP addresses. You get the idea.
actually, as long as you have the administrative access to a site, then you can put on any type of ads or much about anything in it because you have the right to change any location on any web page therein in any site.