Question: We have ants in our house, and nothing I've tried so far has been sufficiently successful. Now I'm considering using ant traps, but only under the kitchen cabinets behind the plinth, a place where neither my cats nor my kids (a baby and a toddler) would ever be able to reach them (no chance of them getting in there). Would that be safe? Are there other aspects we'd need to consider?

Answer: 
I don't think it is overstocked at the moment.   

But these are all schooling fish and they might be happier if the schools are a bit bigger.   

A better mix would be to remove 2 species (eg the tetra's and the barbs) and replace them with the other species (more corys and rainbows for example).   

6-7 fish for a 'school' is usually really them minimum.
Yah. Not overstocked but it will be if you have their appropriate schools. Try two or three schools of the smaller species like zebra danios and turquoise rainbows. You could possibly put some guppies in if you are interested.