Have you tried not running the Beat Detect feature until you've got your music clip into its final position?
Yes, the way I do it is: I first put the music clip into its final position, then I run de Beat Detect, so I can see where the beats are, and then I start to put music clips on the timeline. This music clips are then automaticly made longer or shorter so it fits to the beat detection, but that is what I mind - so till now I was always putting on the beat detection, putting the movie clip on its place so it starts with a beat, but by that is was making the movie clip shorter, so I put the detection again of, making the mopvie clip longer as I like it, and then put the detecetion again on, so I can put another movie clip to start with a beat. And this switching on and off I have to do then all the time so I get the movie clip in a right size, not automaticly made shorter. And my question is, if it is possible to put the beat detection in such a way, that it will not automaticly make my video clip shorter while making it fit between two beat marks?
Again, it sounds like you're saying that you run Beat Detect first, THEN you put your music clips on the timeline.
I'm not sure how you're doing this -- but you should put the music in place first, THEN run Beat Detect.
When "nudging" Audio, or Video, I find it very useful to turn Snap OFF, so that it does not get in the way. Just remember to turn it back ON, for additional editing.
Also, if the Audio, or Video, that you are nudging, is linked, say with a multiplexed file (both Audio & Video), if you want to just nudge one, Alt+click that stream (the Audio, or Video), to temporarily unlink them. When you have finished, as soon as you finish the nudging, those multiplexed streams will automatically relink.