Why waste money on a temporary tank? It's not like they require massive tanks - they just need more than a small tank. And if you are going to keep goldfish, then you should keep a group since they are highly social fish.
Streams don't become a big block of ice, and anywhere that will freeze top to bottom will not have fish frozen in the ice, I can assure you. They do have a good sense of self preservation. Moving water does not freeze, so while the surface may have frozen over, the water below is still flowing.
125 - BGK, chanchito cichlid, pictus cats, silver dollars, palmas bichir
125 - cichlids (severums, bolivian rams, chocolate), rainbows ( turquoise, red), loaches (angelicus, zebra, kuhli and horseface), plecos (BN, RL and clown), denison barbs, tiretrack eel, pearl gouramis, betta
90 - Congo tetras, african knife, upside down cats, spotted ctenopoma, kribensis, delhezzi bichir
2.5 - betta