1) I use gaim without any problems. To find others, use www.google.com/linux,
and search on the keywords 'instant messaging', then start reading.
2) I have two hdds, SuSE on the first, and SourceMage on the second. Booting is not a problem, for either one. I use grub as my bootloader, but lilo would also work. There are other bootloaders which would also do the job. Google up a search on bootloaders; try them; settle on the one that does what you want it to do. The bootloader presents a menu of bootable OSs, you just use the up or down arrow keys to select the one to boot, then press enter.
3) Are you talking about filemanager or web brower? Since you're using Slack, I don't know which filemanager you're using. As far as web browers go, Opera is my favorite, Konqueror is second. Konqueror is also my filemanager.
4) Learn command line operations. I find that there are some things I like to do via point-and-click in a GUI, and some things that I can do faster via command-line. A good tutorial to try first, is Newbies Linux Manual. It's a set of common command line operations, as well as some more advanced considerations. Then, you might consider downloading the Rute Users Tutorial, and LAME (Linux Admisistration Made Easy).