Hi,
1) You'll have no problem getting used to Slackware. As regards normal (non-root) use, you can configure it to whatever way you like anyway, so it should be essentially no different. There are some differences in administration - the /etc/rc.d directory for one - but it's all very straightforward and well documented.
You hear a lot about how hard it's supposed to be to install Slackware. I must admit, it took be a few goes when I first installed it, but I've reinstalled a couple of times since then and never had a problem.
2) Don't know specifically about this, but I bet you can. If you look at the hcl section of this site, you'll probably be able to confirm that. Or search the forums.
3) Installing non-standard packages is the same: configure, make, install. Standard packages use the installpkg program, which pretty much does the same thing for you using tgz files. It's easy, easier than the rpm system.
4) Both available for Slackware.
Two very useful links:
the slackware book and
Linux packages is good for any additional packages you may want.
And, as usual, this site is an excellent resource for help and tips.
Good luck,
John