I'm not sure what type won't outgrow that. All I can tell you is don't get tricked into getting a feeder goldfish since they're small. I rescued one from a bio class once, took it home to my 10 gal and it became a Godzilla sized fish a a few short months. He was very pretty though, with gorgeous fins, especially for the $0.39 that they cost. So as long as you don't mind exchanging them out after some months time you should be fine.
Oh, and you'll want real plants in there. Plastic is great and all but goldfish are meeeeeeeeessssy! (They'll love you all the more for real plants) 
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55gal,260 watt PC(2x65 actinic,2x65 10k daylight,8 lunar LED) 45#LR, 45#LS, 3 maxi-jet 900's on a natural wavemaker timer,phosban reactor, 5 gal fuge (built out of standard 10 gallon tank) with chaeto, grape, and feather caulerpa, sealife systems protein skimmer, wet/dry, 4 blue/green chromi, 1 skunk cleaner shrimp, 2 lyretail anthias, 1 royal gramma,1 true perc 5 mexican turbo snails, assorted dwarf blue leg hermits, astrea snails, and nerite snails |