I wanted to ask wheter it is possible to kill a boss and you don't jump into the next level. So you can put the boss in the middle of your level or perhaps place more bosses in one Level?

You can do this with the Queen Boss but it's not possible with the others.
Just make the Queen fall off the edge without her falling onto any Springs and the level will not end.

Originally Posted by Calvin

I wanted to ask wheter it is possible to kill a boss and you don't jump into the next level. So you can put the boss in the middle of your level or perhaps place more bosses in one Level?

Split the level. Make it so you travel up to the boss room, fight the boss, and the level ends. Then you start the next level in the boss room (with a new path open, obviously), and continue on your way. I've done this in several levels when I want to have a boss in the middle.