I was wondering if core/tut/lab selection is as intense as course selection. I mean not being able to go on solar, some cores/tut/lab filling really fast... Is it the case?
Thanks.

Pretty much. Just keep trying repeatedly to get into the cores/tutorials and labs you want.
Also, unless you have iClicker questions and unless the class is packed to capacity, if a different core is more to your learning style, you can attend that one.

If SOLAR assigns you to a core that conflicts with a required course and you cannot resolve it yourself, you can email the respective department to open up a seat for you in a different core. Same with labs and tutorials.

I think it might actually be just as bad, if not worse :S. You might get conflicts, so you'll have to work those out. However, don't worry too much about conflicts, because those can be resolved by talking to the instructional assistant/lab co-ordinator later on.. And if you have a mock schedule, getting everything you want might take some time and a LOT of SOLAR logins until you can switch into everything you want. Just check it often and hopefully, you'll get into all your preferred sections.