He will be ok in the tub on paper towel but once he poops if all looks good I would put him in something with some cocofibre so he can bury himself, this will help him feel secure. And yes you are right that some of these little guys get stressed over big enclosures where as some are fine. What I do with mine is I start with a 10 gallon and divide it in half, as they grow I gradually move the divider giving them more space until eventually its the full ten gallons. They grow like weeds so they tend to out grow enclosures fast so it doesn't take long for them to be comfortable in full 10 gallons. You are lucky you don't need additional heat, you shouldn't have to hard of time keeping humidity since you don't have an extra heat source to burn off the moisture. For the vitamins I'm not familiar with the Repticalcium but if it has D3 in it you should be fine, but if it doesn't you will need a vitamin that does have both. The Reptivite should be fine though.