Welcome to the forum!
I had kribs and I personally don't like to see them in anything less than 30 gallons, especially in a community tank. The kribs will get really aggressive when spawning, which they will do roughly once per month.
Have a Plan B for the betta (siamese fighting fish) as quite often they do not do well in community tanks.
I agree that you need to get your own test kit. Pick up the API master kit. I wouldn't be surprised if the staff at your store do not know how to correctly do readings on your water. It's happened to many of us.
75: blood parrots, featherfin cat, emperor tetras, turquoise rainbowfish, BN plecos, japanese trapdoor; 46: WCMM, gold inca snails, ghost, bamboo & amano shrimp, kuhli loaches, rummynose & ember tetras, endlers, platies, flame gourami, guppies; 16: pygmy, peppered, loxozonus corys, otos, assassins, RCS ... ~ Research PRIOR to purchase.... ~