In the first instance, what do you mean by 'thickening lines' and which ones are you talking about? Do you mean all of them? Or just certain ones? Leveling slightly might make them darker and give them more visual presence without actually thickening them. Selective color could do the same.
As for the second point, if you want to make your own spiky bubble, try drawing an oval then going to town with the line tool. You generally don't have to, though. Why do you ask?
Balloons like that one are generally halftones themselves. How to get one? Buy a program like Manga Studio and/or Computones. Hell, I'd suggest doing that even now, that way you can 'ink' and tone your drawings.