1. Frozen/thawed is safer for the animal because it eliminates the risk of a conflict between hunter and hunted. Weather or not the snake "enjoys the kill" is apparently a topic of debate. There is a recent thread in the ball python forum which effectively illustrates the risks associated with feeding live. That said, there is nothing inherently wrong with feeding live, it is a husbandry decision. I fed live prey for about a year before switching to frozen/thawed because I decided it was both safer for my snakes and more convenient for me. They (a ball python and a mexican black kingsnake) didn't seem to mind.
2. Substrate is a very common topic. There are lots of options... Aspen and newspaper are probably the most popular. Bark, mulch and coconut husk (bed a beast, eco earth) are gaining popularity. Sand is not often discussed, and having never used it myself, I don't have an opinion on its viability. I'd suggest a Google search.
3. I've done a lot of reading on this, and most internet sources say 20 gallon (or 30 X 13 X 13"
is the minimum for most kingsnakes... but why do the minimum? 20 gallons is awfully small if you ask me.
4. The issue of cohabbing is often debated on this forum. Kingsnakes do have cannibalistic tendencies, so if you cohab them, there is a possibility that under the right circumstances, one snake may eat the other. Some say these circumstances are avoidable, hence the debate.
5. Two hides are most often recommended, one on the cooler end of the heat gradient and one on the warm end. This allows the snake to thermoregulate and feel secure both when warming and cooling. Some offer only one hide on the cool end, which forces the snake to choose between basking and hiding.
I sympathize with new people, as there is a lot of conflicting information out there. I think reading forum posts, asking questions and seeking out information from a variety of sources are all great ways of educating yourself- of course, keeping a snake is the most educational of all.