God Please No...(also - this tank is only 6 gallons - so especially no).
That type of a tank is okay for freshwater fish. And you *might* be able to get away with adding a couple saltwater *fish* to a tank like that.
But as soon as you start fudging around with inverts and coral the thing is going to crash. The coral/inverts won't survive I don't personally think. Corals need specific lighting and many inverts do better with a really established filter - something you won't really get with a HOB.
If you want a nano reef, get something like a Bio-Cube by coralife OR an Evo tank by Fluval.
These tanks have the filters built into the back of the tanks themselves along with space for a protein skimmer and UV sterilizer. I think you could probably fit a heater in that space too.
These kits also come with lighting that's adequate for corals/reefs.