because you have failed to grasp the basic, yet ultimately flawed, 1" per gallon rule (50 cardinals???) new keepers generally follow then i have to add to some of the other question marks raised about your options too.
without prior saltwater experience, and from your list it looks like your lacking general fishkeeping experience at that, there is a high chance you will run into difficulty on a volume as small as 10 gallons with general marine inverts never mind specialist reef, octopus or seahorses.
have you researched at all prior to compliling your potential stock?
if not then i strongly recommend you do before trying to decide which to put. you need to know a little about something before diving head first into it. you will likely save money too because failure will be less likely. apologies if you feel thats a harsh or unfair assessment.