If they are males and if you have A very good filter system and if you keep up with maintenance, you might be okay. They need space. I read that each koi should have 1000 gallons. If your equipment did a super cleanup job, you might be okay.

I think I would go for the larger size.

Could you start with 2 and see how things go?

Is this a new pond? If it is, use a few comets to get the pond started before adding expensive koi. That way you won't be sick from the shock of loss to new pond set up.

10 gallons per inch of Koi as a general rule of thumb.

Koiphen or Simi Koi, both have extensive data. Koiphen has an active forum.

Koi grow so plan accordingly.

1000 gallons with 2-3 koi is more than doable. I run just at 1000 gallons and have 4. I filter with 2 inline pressure filters, and a UV light. No issues with water clarity, or condition. I built an above ground pond due to inability to dig into the soil. 30" deep, 8.5' x 6.5'.

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