I answered one of your previous questions today and brought up the undergravel filter there.
As far as an undergravel, people do use them in saltwater tanks, no denying that. Many have used them with success.
I don't use them (personal choice) and have had success without them. What I consider their major drawbacks are that with the size of the substrate used in some marine taks, the undergravel plates can have the substrate get stuck in the holes that are supposed to have the water pass through. Add any organic debris and you can get a fair sized area clogged or trapped beneath the filter where it's harder to remove with a siphon and the detritovores (snails, hermits, seastars) can't reach it, so as it sits there, your nitrates build. If enough of the filter plate and beneath gets clogged, anaerobic areas may form where hydrogen sulfide gas can build up. This is toxic to fish and other organisms in the tank.
That's why I said that if you use the undergravel, using the powerheads with the flow reversed as you'd planned was the way to go. By reversing the flow, you're blowing water down the tubes where it will blow any detritus upward to reduce clogging and trapped organic material to prevent the formation of hydrogen sulfide. But it will blow any dust from your substrate around as well.
I'll forewarn you about the Skilter here, too. I've bought two of these when they first came out with plans to use them on 10 gallon quarantine tanks. Both leaked terribly where the motor connects to the plastic filter body. I tried wrapping them silicon tape (used to prevent leaks) and after a few days, they started leaking again. Also, the efficiency of a skimmer is related to the amount of time that the water is in contact with the air bubbles. A good skimmer column should be the height of your tank or longer. A Skilter is maybe 1/3 to 1/2 the height of your tank (you didn't specify 20 standard or long). They will remove some of the organics, but for the price you pay for them, and their tendency to leak, I would rather spend a little more and get something that does a better job and keeps my floor dry.
Apart from that bad news, you appear to have a good set-up. As long as the substrate is under 4mm average size, this still fits in the range of sand - probably small or broken shell material to help buffer the pH in the tank. The heater is a good wattage for the tank (5 watts/gallon) unless where you're keeping it is more than 10 degrees below the tank temperature you want to maintain. And you've actually got more than enough filtration power (in gph).
For fish, I'd suggest just two clowns and buy them as juveniles. This way you'll be more likely to end up with a mated pair. A third would likely be rejected and harrassed by the other two once they pair up. Gobies and shrimp are fine additions and you might want to add a few snails and hermit crabs for cleaning. And you're more than likely going to get some other additions on the live rock.
I put a few websites below with more info on undergravel filters in marine tanks, showing both good and bad points of using them.
Good luck with your tank!
ADDITION: Good that you're planning ahead for an equipment upgrade. As far as the DIY trickle filter - here are some plans: http://www.sydneycichlid.com/content/?page_id=15
Don't know if you're planning to use this on the 20, or wait till you go up to a 55. Is your present tank drilled for gravity feed? If not you'll need a over-the-tank design, or risk cracking the glass if you try to drill. You might want to wait an a trickle filter until you buy the 55 - and buy one already predrilled.