Could work, but I wouldn't mix rams and kribs together. Choose one. Too many dwarf cichlids in a small tank will end up with fights over territory. Since dwarf cichlids like these will claim about 30-45 cm radius around their "nest", a 20 gallon tank will be big enough for one fish (or pair).
Actually, I don't believe in mixing labyrinth fish with cichlids at all. They aren't a good combination except when handled right. The cichlids and labyrinth fish have similar demands but conflicting temperaments, and usually the cichlids hammer the labyrinth fish. This is certainly true with climbing perch versus African cichlids of similar size, and tends to be true with gouramis and "peaceful" cichlids. There are combinations that work (e.g., lace gouramis and angelfish) but I wouldn't predict dwarf gouramis with rams or kribs would be 100% safe.
20 gallons is (in my opinion) too small for a pearl/lace gourami -- they get to at least 10 cm, and need a fair amount of space. Better stick with small species. I have a hearty distrust of dwarf gouramis UNLESS obtained from amateur breeders. Too many dwarf gouramis are sick with bacterial/viral diseases that can't be cured.
Kribs work best with fast, midwater fish. Rainbows would be ideal. Both would thrive in pH 7, slightly soft to moderately hard water. So would sturdy tetras, such as bleeding heart tetras or x-ray tetras. Hatchetfish, small barbs, or glassfish would be good, too. Neons and cardinals not so good because they stay close to the bottom and tend to be a bit dozy.
Corydoras are a crap shoot with dwarf cichlids. Worst case: the cories have their eyes bitten off. Personally, I'd skip Corydoras if you wanted to keep dwarf cichlids. Some type of small armoured catfish such as Ancistrus sp. would be a much better bet. So would a dwarf upside-down catfish (though single specimens tend to be shy, and 20 gallons is too small for the preferable trio or more). Small loaches might be one alternative worth considering. If you want to breed the kribs, leave out the catfish altogether.
Your main problem is 20 gallons isn't much space, and provides limited swimming space for active fish like rainbows, tetras, or barbs. With relatively small tanks such as this, you can often have much more fun picking a pair of dwarf cichlids and embarking on a breeding project. You could also do the same with some type of gourami, though breeding gouramis is MUCH more difficult. But paradisefish or one of the more unusal Betta species could be very worthwhile and entertaining.
Otherwise, if you just want "pretty", go for a planted tank and install small, not-too-active fishes that will provide bright colours, for example Endler guppies, neon tetras, or bumblebee gobies.
Cheers, Neale
In a 20gal (us):
2 fm Kribs
1 pr blue rams or a few cory cats (whichever you think is best)
a trio of rainbowfish
1 dwarf gourami or pearl gourami
