Learn Finnish: Minkälainen pelle sä oot, ja ootko sä myös akrobaatti? - What kind of clown are you, and are you also an acrobat?

Both words mean "what kind of" and are completely interchangeable in meaning. minkälainen is built from the genitive of mikä plus the suffix -lainen, while millainen is slightly shorter and often preferred in formal writing. You can use either one here to ask what type of clown the person is.

I recognize sä, but what verb form is oot and ootko?

These are very common spoken Finnish (puhekieli) forms of the verb olla (to be). oot is the spoken version of olet (you are). To make it a yes/no question ("are you?"), you simply attach the question particle -ko to the spoken verb, making it ootko instead of the standard written oletko.

In the second half of the sentence, why does ootko come before sä?

In Finnish yes/no questions, the word carrying the question particle -ko or -kö must be placed at the very beginning of the question clause. Even though the normal statement word order is sä oot, adding the question particle forces the verb to the front, resulting in ootko sä.

Why are pelle (clown) and akrobaatti (acrobat) in the basic nominative case instead of the partitive?