Why "on" and not "ovat" for six cats?
it's because of the numeral. if there is a numeral then the verb is in singular (yes, i know, it's a bit weird :D)
Not so weird, it's the same in Welsh!
Maybe because the subject of the sentence is a numeral that is in the nominative case and singular, and "tuhmaa kissaa" is the numeral's definition in the partitive case (so ony the numeral here affect the verb 'to be').
Exactly! The concept is something like "six of naughty cats is...". In most Slavic languages there is the same idea, although the partitive concept is expressed by the genitive form.
What’s the difference between "ainakin" and "vähintään"?
I have the same question and google gives me this: https://hinative.com/en-US/questions/14468818
Why tuhmaa kissaa and not tuhmaist kissaa like with other adjectives after a number?
"Tuhmaa" is the singular partitive form of "tuhma".
Which adjectives get a form like "tuhmaist"? If you meant forms like "valkoista", "iloista" etc. those are words that have a -nen at the end in their nominative form, "valkoinen", "iloinen".