1. Slack does not come with as advanced dependency checker as other distros, although swaret is pretty popular. You won't really need to worry about this during the initial install, but if you start adding apps, it may become an issue. I'd also suggest looking at http://www.linuxpackages.net/
because that site frequently will list (and have links) for all dependent packages that are associated with the main app you want to install.
2. Not sure why it would make any difference why it matters which tool you use to partition your drive. If you've got, for example, an 80G drive and create a 40G parition, a 25G, and a 15G, you'll end up with a 40/25/15 partition scheme no matter whether you use cfdisk, fdisk, Partition Magic, or whatever. Am I overlooking something important? As for the sequence of steps, my recommendation would be to do the partitioning before you start the install, and then during the install you'll have the option of either formatting the partition or not. If I recall, and as spurious indicates, the format options are ext2, ext3, or reiserfs. If you want to use a different format, I'd perform the formatting prior to beginning the install, and then at the point Slack asks if you want to format the partition, answer No.
3. Like spurious, I don't know. -- J.W.