If I do only one kind of crime everytime does the success chance decrease?
And Don Seladon wants I rob 15 people. Are all crimes considered ''rob people'' or there is a specific crime to commit?
It really depends. When you commit one crime and you have too much experience, it may happen that you will be more likely caught or that task will fail, but it really depends. Every crime is a little bit different, but in general more exp = higher rewards, but some crimes also have higher chance you will fail. You should try different crimes.
Its pickpocketing task, you have to successfully rob someone. If you fail, it doesn't count.