The flow rates on canisters are with no media in the canister so once loaded with media you can expect the flow rate to be a lot less than 1400. That being said if you are starting with juvies in a tank that size I would say 1 would be fine, you can always add more filtration if needed later on. 20 dems is a good num To start with and 10 or so yellows as the yellows usually breed like rabbits so you will prob find your numbers will increase once they hit breeding age. As far as aquascaping that comes down to personal preference. Both prefer rocks so a couple of piles at each end of the tank to help define territories is usually a good start.
What ph is ya water, mid 7's to 8 is what most people aim for.