Let's start with tex and latex. This is a very powerful (and space consuming) word processor. If you don't know it, you don't need it
The red packages are those who have been updated via YOU. It simply means that the version you have installed is more actual than the version on your installation source. But that does not mean it's necessary.
Most packages are categorized by function. You can select those groups in the 'Filter' tab of Yast. Additionally there are short descriptions of the selected package in the lower right window.
It would have been better to start with a minimal system and adding the software you need. It is more complicated to decide what you do not need.
There is a nice applet in System -> File System called KDirStat. This shows a graphical overview of the disk usage. I use this to get rid of space wasting files from my /home. But this should be done very carefully with system files (if at all).