I seriously dont advise it...
Id be a hypocrite saying more than that, I had 3 baby koi that were barely more than 1 - 1.5", they'd have been eaten in my pond as there were no plants.
I had the three of them in a Roma 125 with an EX700 Tetratec filter on it. Within 4 weeks the fish were closer to 3" and too big... especially with that growth rate.
The average temperature in the flat was 18-20 deg C, bloody cold but we had no heating at that point and it was over winter we had them in the tank.
I kept them in there for 6 weeks in total, put an EX600 external on there too, they are SERIOUSLY messy and absolutely hate being on view all the time, they hid as much as they could and freaked if you walked past the tank.
Its also far too tempting to keep them in the tank too long...
I have a customer with two mirror carp in the tank and i even said it was awful situation, they are 20 years old and i couldnt even judge how big they were, stunted for sure but at least 2ft-2.5ft and very...'arched' spines.
The tank was 7 x 2.5 x 2.5 and cost the guy £3K to buy, sort out heating, filtration and lighting and he only had it a year and they grew (they'd been kept in a smaller preform pond *cringe*.
It didnt matter what i said to him but i think the horrified look on my face when he showed me photos told him far more...