Well, it depends on what size tank ur gonna put em in. I have 5, but i have a well planted fish tank, with alot of height, and its a 3ft'er. So any fish feeling picked on can hide away and escape its bully. lol
It depends, different ppl see it differently to me. But i have no trouble with more than a pair, but i would wait and c what other ppl say.
It depends entirely on what size tank. They ARE compatible with the danios but only if they'll not be over-crowded. Keep in mind that both danios and pearls preffer the upper layers as well.
The one in your sig.? If it is, and you say you're having trouble with it, I would wait till everything is stable and then concentrate on building up your tetra shoal to 6, get at least 2 more cories and bring your danio number to 5. That would leave you just about enough room for a single gourami but you'd be better off with one of the smaller ones as getting a pearl could mean you need to do some extra maintaining of the tank.
If all you'll be getting is the 6 danios, a trio of pearls should work. However, 20 gallons is a little less than I would reccomend - particularly if you have no previous experience with gouramies. It may be better to go with a smaller species such as colisa chuna/sota or sparkling or croaking gouramies or just go for a single pearl. There's also the option of getting a pearl AND some croaking or sparkling gouramies.
Whatever you decide, keep track of your water parameters and watch your fish closely. That way, you'll spot the first signs of trouble before they become a problem and you'll be able to keep all your fish and tank healthy.