personally I wouldn't mess with the mantis unless you have at least half inch thick glass. Those things can destroy a tank as easily as a .22 caliber rifle. Octopus is going to be incredibly difficult. If you are just getting into the hobby, I would aim you away from such elaborate live stock. They are poisonous and need special treatment in multiple ways. As for lion fish, you'd be keeping them in a very small enclosure for that type of fish. They deserve better - bigger by far. Keep it simple. You only have 14 gallons to work with. It'll be prettier and work better if you don't try to go so extreme. IMHO.