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.
So, I'm going to slightly disagree with the idea to keep AND add to the rainbows pop. This is one of my favorite breeds of freshwater community fish too, so although my heart says keep those beauties, my experience says differently.

They get about 4" (10 cm) in length and need a decent sized school to be happy. It's said that if you have a proper school a 50+ gallon (190+ liters) tank is what's necessary. Volume isn't the issue with these guys, it's the length of the tank that's needed for their swimming patterns. They are lateral swimmers, middle dwellers, and need the length of 48" (around 120 cm) of more unfortunately.

I'd say remove the two turquoise rainbows (especially since they'll get the biggest), two more zebra danios and add two more corys. I think your 5 cherry barb and 5 serpae tetras WILL be fine the way they are. Since it's a 30 gallon, and since your question shows your concern for overcrowding. Good luck and have fun!