Sorry but I don't agree. You can use "tum" with a group of people. The difference between "tum" and "aap" is that "aap" is more formal than "tum", but you can both use them for a single person or a group. (As in English "you guys", you can say "tum log" log=people, to show that you speak to several people, if you really want to be clear).
"haiN" is used with "aap", and "ho" with "tum". "hai" is used with "tu" which is very informal (very close relative / lover and younger than you / can be insultive) and can only refer to a single person. So : tum ho , aap hain = plural or singular. tu hai = singular, very informal.
In the sentences "तुम कैसे हो?" = one man or several people, not formal, " तुम कैसी हो?"= one woman or several women, not formal. I hope that was clear.