Goldfish can get 10”-12” on average (I’ve heard and seen them get bigger) in cooler waters they can be slow growing but it isn’t necessarily healthy for them. I would recommend a bigger tank for them as well and keep their water at 72-78F or 22.2-25.6C. (Again while they can stand a wide range of temperature doesn’t mean that tropical water is healthy or cold water as healthy).
But if they’re small it should be OK for a couple of months. Just keep an eye on growth and water parameters to know when to switch to a larger tank. Shubunkin and comet goldfish are fast swimming and need a lot of room to move around comfortably so also keep that in mind when getting a bigger tank.
Fan tails are easier, they are slow and just need big enough tanks to turn around in and enough liters to handle their bioload. (Again at full length that is 10” + or 25.4 cm +)