The mods in the Library are all pretty safe to use. I currently have 44 installed and running with no problem (yikes didn't realize it was that many!). You should know that KSP has a memory limit and that exceeding this limit (huge mods, lots of mods, lots of ships, graphics settings too high, etc.) will cause KSP to crash ingame or even prevent loading. When you encounter these problems install and run Active Texture Management.