Great info, thanks. Now here's my two cents worth
First let me welcome you to AA and commend you for reading up before actually setting up your tank! Keep asking questions, the people here are friendly and knowledgable.
Probably use mail order LS but I dont know if it arrives wet or dry.
If you mean the LS
that's sold by the pound and must be shipped overnight or two-day air, it will arrive wet and hopefully be full of critters. This is what you want.
Not for sure I dont know what a good depth would be for a 55gal
is defined as 4" - 6" sand depth of sugar sized particles. A great article on DSB
's can be found here
Put together a list of fish you would like to have and post it as a new topic. You are sure to get lots of advice. Everyone has their favorites. several of the online retailers have compatibility charts, but after checking those you should also post here for comments from people who might have or have tried that combination of livestock.
Good luck with your set up. Take your time, go slow and and listen to the advice of the more experienced folks here and you should have few problems.