Java fern - You could get 2-3 small plants, or 1 big one which i think would look better. I'd suggest buying online for plants, they are alot better in quality.
CO2 - I thought that this CO2 thing was alittle complicated but, it isn't atall. I have 3 Diy CO2 bottles running on my 20 gallon, and they work fine. What i did was buy the nutrafin kit for £15, came with a plastic grey bottle, airline, ladder(diffuser), and dry mix. I would not bother with this, waste of money IMO. Do what alot of people do and make your own. All you need is a normal plastic pop bottle, airline, one way valve(optional), diffuser. I have recently bought a ceramic glass diffuser off ebay that connects to the end of the airline. The whole point in the diffuser is to dispurse the CO2 into your tank. Cost me £4.99.
The mixture is simple, 2 cups of granulated sugar, active dry yeast and warm water. Every fishkeepers methods of mixing these together are different, just experiment with it.
Fish - The khuli's you won't even see because you have sand, and they only come out at night. I had 3 in my 20gallon for 3 years which i didnt even see until i found them dead after the 3 years.
If it was me id still prefer to have angels instead of rainbows, thats just me though.
Kribs and rams can work aslong as they have their seperate territory's which they will have. The cory's would look great, especially in a large group.
If you were going to get rams i would ask the supplier if you can have 5 rams and then once they have formed 1 pair then you will return the remaining fish. You can also do this with angelfish if you wanted.
Keep us posted mate, Ill be watching.