I think almost any fish that hangs out near the top, is bound to be scared by noises. Danios and rasboras, which hug the top and swim actively, may not be, but hatchetfish, butterfly fish, gouramis, golden wonder killies, bettas and other fish which just lie in wait near the waterline, are all going to be startled by a hand in the tank, a sudden flick of the lights, or a loud knock on the glass. It is something to consider, but I wouldn't say a deal breaker.
I can see you are trying to be very conservative with your stocking. I can tell you from experience, having helped many people over the years with their various aquariums, that your sister (and anyone else involved with the aquarium) is bound to find this stocking level sparse and complain about how empty the aquarium seems
If you are concerned about bioload, maybe add 6 white clouds to your 6 cherry barbs. They will add very little to the bioload but will add considerable interest 
I used to have just 6 bronze cories in my 29 gallon tank, but recently added 6 peppered cories (they were mislabeled at Walmart and were only $1.25 each!) It's made a HUGE difference in their behavior, and I can't see that it's affected the bioload. Cories are like loaches for the most part, the more the merrier.
I can see you are trying to be very conservative with your stocking. I can tell you from experience, having helped many people over the years with their various aquariums, that your sister (and anyone else involved with the aquarium) is bound to find this stocking level sparse and complain about how empty the aquarium seems


I used to have just 6 bronze cories in my 29 gallon tank, but recently added 6 peppered cories (they were mislabeled at Walmart and were only $1.25 each!) It's made a HUGE difference in their behavior, and I can't see that it's affected the bioload. Cories are like loaches for the most part, the more the merrier.