What's the difference between velicí and velcí? It did accept velcí for me so is it always interchangeable? I think I've read it before but can't find it
They are completely interchangeable.
If any sentence was rejected and you want to know why, you must report the complete sentence.
Could there be other answers to this "Jsou klucí velici" or using hoch?
"Ti/tamti/tamhleti chlapci/kluci/hoši jsou velicí". 'Jsou' shouldn't stand at the beginning unless you want to ask a question.
Why "ti chlapci" instead of "to chlapci?"
Because chlapci are maculine animate. Do not confuse this with "to jsou"/"jsou to" that can be used with anything of any gender.
'Ty kluci jsou velici' - does this make sense?
It makes sense, but "ty kluci" is common Czech, not standard. Since this course teaches standard Czech, you have to use "Ti kluci jsou velicí".
Notice how the soft ending (-i) occurs in each word: ten veliký kluk -> ti velicí kluci
How many words are used for 'boys' - kluk, hosi, chcapci and more??
Are they used for different times??