Idk about your lighting and HC or dwarf hairgrass, but your bubble rate would depend on the amount of light and types of plants you have, so that's a difficult question to answer. It takes a bit of adjusting to figure out what's best for your tank. The goal is to get 30ppm, but in actuality, you should gradually turn it up (over the course of several days at least) until you notice signs of respiratory distress in your fish, then turn it down just a little bit. Start the CO2 around 1hr before your light comes on and it should be okay to turn it off 1hr before lights turn off.
You shouldn't need an air stone if you have fish. The surface agitation that the bubbles create will actually outgas the dissolved CO2 quicker.
As far as substrate goes, fluorite black should be fine. I have fluorite dark in my 30 gallon and it's worked great for over a year now. I don't even really thing that the laterite is necessary.
The dry start method for growing a carpet isn't completely necessary. Search "dry start method" and read up on it. I'm considering doing DSM on one of my new tanks, but I don't have any experience with it yet. I grew a glossostigma carpet in my 30 gallon without using DSM. It might take longer for the carpet to fill in, but it's not really a big deal. There's only certain plants you can do it with too, although HC is one of them.
Hope this helps some.