compatibility and tank size

Question
I have a 30 gallon tank with 4 rosy barbs (2 long fin females, 1 long-fin male, 1 regular male) 1 shubukin goldfish about 2-3 inches(I think), a plecostamus about 2 inches, and a long skinny shaped catfish, about 2 inches. I have 10 gallon tank with 6 guppies and 5 platies, a small fantail goldfish (they are excellent scavengers) and a small pleco. I have a 5 gallon tank that I was using for 3 long-fin blue danios, I bought a crab recently (fiddler I think) and was told that it would be fine with any fresh water fish (probably because they wanted the sale). Any how, the problem is that after researching the crabs found they need brackish water and a rock to reach the surface. I want to make the changes to the 5 gallon for my new little crab but that would not make good conditions for Danios, apart from the salt, they will not have much room once I add a rock and lower the water level.
Okay, the question is which tank should I add the Danio's to, or will they be incompatible with both? Also do you think any of the tanks are already overcrowded, my husband thinks that they are fine, but I'm not so sure. Are there any incompatibility issues with the other tankmates?

Answer
HiYa!

Ok I can't comment on whether your tanks are overstocked or not because having an overstocked tank is not just dependent on how many fish you have in the tank. It also depends on what filtration you have, and how often you clean your tank and do regular maintenance on it. Alot of people have what looks to be overstocked tanks when you look at them but they may have 2 or 3 filters on the tank and do regular 25% water changes once or even twice a week.

I would put your Danio's in your 30 gallon tank. My next question is, your Plecostamus that you have, what type of Plecostamus are they eg Bristlenose, Sailfin etc etc....

Cheers! Rach