If you wonder about a snail's viability, pick it up and take a good whiff. If you are still standing, put it back in the tank
. When you order more, put them in the center of a plastic container (the microwave meal kind with a flat bottom work well) and put in a small amount of tank water in with them. As they move to the edge, you can put them in your tank. It there are any left in the center after a couple of hours, see above.
While you are playing with the tank (and after you get your mat), you can net out the white floaty stuff. If you don't have one, I would recommend that you buy a brine shrimp net (Tom's also makes a very fine (as in hole size) net that is entirely plastic, is very inexpensive and is my personal favorite). I find that the brine net and/or the soft one by Tom's do well for removing ink if (as is usually the case) you are in front of the aquarium when it happens. A second item you should have on hand is a decent turkey baster. This will be helpful for disrupting your bottom substrate, blowing off your live rock during clean-up and is also useful for collecting ink that is mucusy (thin ink will be difficult to catch but will be handled by your filtration).
There are several threads on acclimating an octopus and several things you should read about acclimation in general before you are ready for a sensitive creature. I tend to acclimate my octos a little differently than the best recommendations on the site and the site methods may be best for a novice.
However, if you don't find enough in the care section, PM me and I will give you the link to my write up for the website I webmaster for some general considerations (not octo specific).
A good source specific to octopus acclimation (with special octo specific tips) is
the book written by TONMO's Nancy and Colin.