1 - Yes, I'll do that. That's my purpose behind soaking the soil.
2. I thought that the clay was needed to provide additional nutrients for the plants? Or did I misunderstand? (likely - chemistry was never my strong suit)
3. I was going for a loose cap because I'm getting goldfish, and figured that the slightly larger cap would be less likely to be disturbed by them than sand...but can certainly be convinced that just a sand cap would be best.
And re: tank, planted aquarium, with an Eheim canister filter, and stocked with goldfish and generally goldfish-friendly plants...