Is kouta played on tsugaru shamisen? If it is not traditionally played on a tsugaru can it be?
you can pretty much play anything on a tsugaru, but generally kouta is played on a Hosozo.
It seems to me that the futozao used for Tsugaru is kind of like a grand piano whereas the smaller sizes are comparatively like a harpsichord.
So whereas you can pretty comfortably play harpsichord music on a piano
it`s not quite the same in reverse (try playing a Chopin etude on a harpsichord… it is very painful and unnatural not to mention probably impossible.)
So it seems pretty much most music written for hosozao or chuzao can be easily translated to futozao but the agressive music which originated on futozao appears weak and somewhat downgraded when attempted on smaller ones.
I thought there’s a specific kind of shamisen when playing Kouta?
like… Kouta shamisen? rly big and stuff :DD
I think they are quite similar to nagauta shamisen, or are just played on a nagauta shamisen with the right bridge etc. I don’t know any kouta but I have heard they are often played without the bachi. I would imagine they were small as they are usually played indoors. Heres a kouta shamisen.
Kevin nailed it on the head. Shamisen aren’t the most “backwards compatible.”
A powerful car can always slow down to the speed of a smaller car, but it’s difficult for the smaller car put out the same umph as the fast car.
That’s not to say that you can’t play tsugaru pieces on smaller shamisen. If a nagauta shamisen is all you can afford, then by all means, use it for any songs you want to play. Bousama did that as well, as it was all they had. It’s just a tsugaru shamisen will make the sound of those pieces more… fulfilling. 
(like revving a beast of a car)
Interesting. The kouta shamisen on e-kameya says the sao is “chuuzao”, not “hosozao”. That’s pretty interesting. I don’t know much about kouta style, but I wouldn’t have though it would use a chuuzao size sao. Also, the dou is the same size as minyou shamisen.