Being new to this site, I don't want to cause any hard feelings, but.....
While I will agree that bigger is better (with regards to saltwater keeping, especially for the newcomer), I do have to disagree that a smaller tank shouldn't be tried. Especially if you have the advantage of being a freshwater keeper first.
My very first salt tank was a 29g that was very successful and I have known others who started even smaller with good results. I have at the moment a 20g and a 55g and to be honest, I'm thinking of starting up another 20g. (Please someone, find a cure for this fishkeeping disease!!) I've also had in the past a 5g tank that I tore down only because it was so boring....I was growing some Mermen's shaving brush algae in there and was very, very limited on what I could keep in there with it.....(or so I thought!)
Definitly go with a deep sandbed. A properly set up sandbed will help to lower nitrates. Do not rely on your substrate to keep your pH levels up. The ability of argonite to dissolve starts at a pH of 7.5 or below which is too low for marine animals.
UG's are definitely a no-no as too much detrius is trapped. 2 HOB filters are too much, but I would keep one as they are much easier to keep clean and you can run carbon in there. 1 or 2 powerheads for water movement. No sponge filters on these as they will slow down the flow and also they are much harder to get to so you will be less likely to clean them. (Unless you are trying to raise any kind of fish larvae or copepods in which case this would be the way to go).
Live rock is a must, even if you have no desire to keep corals. It gives the benefical bacteria a place to live, provides hiding spots, gives the fish something to graze on and gives the tank a natural look. I would not use driftwood as it can sometimes lower the pH of water, plus it will stain the water and you will want it clear. Fake plants are up to you....
In a shallow tank like a 10g, regular flourescents will work fine if all you are interested in are some macro algaes, and fish. Inverts like featherdusters, snails and crabs don't require any special lighting either. If you desire corals then yes, PC's are the way to go for a tank that size.
A saltwater tank has to be cycled, but use the fishless method. Simply drop a raw piece of fish or shrimp in the tank and keep watch with your test kits.
A temperature of 80º to 82ºis now recommended (Dr. Ron Shimek @ reefcentral.com), and if you intend to keep ANY inverts like snails, hermit crabs, mushroom corals, etc....it's best to keep the salinity at 1.025. A pH between 8.0 to 8.4 is needed.
The biggest problem with small saltwater tanks is evaporation. When water evaporates in a salt tank, only the water goes, the salt stays behind. So if you lose a lot of water, your salinity will go up. It's mainly the fluctuation of this that causes problems (unless of course the salinity is allowed to go too high). I have found several ways to make this easier for me to deal with. The first is to use a hang on heater. The kind that has the water mark....you know, where the water level must be? IF you make sure your salinity is where it is supposed to be 1.025 and IF you make sure your water level is at or slightly above this water mark, THEN you only need to glance at this daily to see if you need to add top off water (no salt) or not. You can also use a piece of masking tape on the outside of the tank to do this. Once you have your salinity level correct, just put a piece of tape in a discreet spot level with the water level. Again, a simple glance is all that is needed to see if water needs to be added.
The next biggest problem is temperature fluctuation. Keep your thermometer where you can easily see it. A good heater will keep the tank warm, but in the summer, unless you have air conditioning, your tank may over heat. A fan aimed across the waters surface will help in this matter.
Another big problem is pH fluctuations. If your local tap water is good, and if your normal tap pH is at 7.0 or above, then your saltwater mix will keep the pH at it's proper level. (That and regular water changes) But if your local water is crap and you must use RO water and add buffers to it, it would be a good idea to check your pH in the middle of the week to make sure it isn't falling. (At least for the first couple of months until you get the feel of your tank).
Fish for a 10g must be small at adult size and no more than 2. Percula clowns are good, you could go with a pair, or one of them and a yellow tail damsel. Royal grammas stay small, or you could try bangaii cardinalfish.....just be careful not to overfeed. If you really wanted to go with seahorses, you would be better off looking up their requirements at
www.seahorse.org Many species are now captive bred and trained to eat frozen foods (Ocean Rider is one of the breeders). Don't get me wrong, they are still difficult to keep, but if you were determined and dedicated it can be done.
Again, I agree that a bigger tank is better, but yet I understand many peoples desire to try a marine tank that maybe don't have the space or money for a larger tank.
If people are willing to put the effort into the smaller tank then they can have a slice of nature too.....
Just my 2¢