I will agree about the "shark" not being suitable for your tank. They average 6 - 8 inches long when full grown, and can be VERY aggressive, especially in a smaller tank such as yours. I have one right now that is approaching the 5 inch mark, and he has a 40 gallon tank all to himself. Eventually he'll go into the 180, but for now, he's alone because he's fond of eating and harrassing his tank mates. This is normal behavior.
The same thing is going to apply with any of the gouramis if you choose to add neon or cardinal tetras to the tank. Keep in mind that neons are more tolerant of higher pH, cardinals can be tricky. All of the gouramis, even the "peaceful" ones will be too aggressive to keep with fish like neons.
Gold Nugget plecos are awesome, but will get way too big for a 29 gallon. The last one I had was about 10 inches and still growing. There wouldn't be enough space or food supply in a 20 gallon for a gold nugget.
The bulldog pleco, aka rubbernose pleco... another of my favorites. These guys stay smaller, but I have seen them up to 5 inches already. Two things to note about these: they are very sensitive to water quality and they eat A LOT! If you have live plants, expect these guys to devour the plants in a short period of time.
The pitbull pleco would be a better option, but I would make that the only pleco in that size of a tank because of their impressive eating habits.
If you want multiple "plecos" in the tank, even tho its not a pleco, what about otocinclus? These top out at 4 inches, are slow growing, eat A LOT, and won't bother your live plants or fish? The otos are cool, peaceful, and pretty tolerant of water conditions if acclimated properly. Otos would be the first I'd suggest in a smaller tank with something like neons.
As for rummynose tetras, again you're talking about a "difficult" fish to keep. IF you can get them to settle in, they tend to thrive, but the hard part is getting them to settle in to begin with. Rummynose will be less tolerant of condition changes, much more prone to stress, and need extremely passive tank mates, such as the neons. I'm not sure I'd mix the rummynose with fiesty danios, as the stress of the chasing would be a bit much for them.
May I make some suggestions for you to consider? I am considering the size of your tank, the idea that you want bright and colorful fish, peaceful enough for neons and still able to hold their own with danios (watch the danios with the neons, too).
coral platys Scarlet Badis
or Badis badis
marble hatchet fish (specifically the marbles due to aggression levels)
green fire tetras
botia loach (lots of types of botia to choose from)
Now, my other suggestion would be to skip the neons and/or caradinals, and work with fish that are better able to hold their own with those danios. Here is a list of what I'd suggest based on your dainos and the size of the tank: Cherry Barb
neon Rosy Barb Odessa Barb
(the males are extremely bright and colorful) Dwarf Gourami
(like the thicklips that you mentioned)
neon dwarf rainbows
These are only SOME of the possibilities out there for you, but I hope this helps a little. These are all "easier" fish to keep than some of what you had named, but still colorful and active, compatible with the danios you have currently. I would suggest watching stocking limits, as 29 gallons still won't hold "a lot" of fish. Best to choose just a few species, and if you need more ideas, I can give those, too. If you have more questions, please feel free to ask. I will watch this thread to try to keep up on it faster. Sorry for my delay in getting here.