In my 15 gallon Mantis Reef I have kept things pretty simple.
HOB filter
HOB skimmer
Reef Lighting
Live sand
Live rock
frags, and a Hippo Clam.
HOB filter. Two ways to go, one is to run it as normal with filter pads, the other is to fill it full of Live Rock rubble, alla FOWLR. Many also turn HOB filters into mini fuges. My personal favorite is the Aquarium Systems Millennium 3000 which has two seperate chambers, one for LR and the other for a fuge.
HOB skimmer, I am very impressed with my Tunze NANO skimmer. If you are thinking of going and making a florida Biotope with Thorny Oystuers then I would skip the skimmer, as I have found that aggressive skimming can starve oystuers.
Because of there small size lights heating the water can greatly affect the water temp, so I would go with T-5 lighting. The extra shallow depth of the tank will allow you to keep just about anything.
As to tank mates that will depend on your mantis. With my N. wennera I have a pair of yellow tailed cromis and a saphire blue damsel.