A sponge filter is probably your absolute safest filter for that size. There's no way to get sucked in - though I have never had a fish get sucked into a filter.
That filter will be fine.So say I get a sponge filter like this (http://www.amazon.com/Jardin-Aquari...upplies_2?ie=UTF8&refRID=18QV9ABN881XPJ1531GV) and a Tetra Whisper Air Pump suitable for up to 10 gallons, would that work? Do you think an up to 10 gallon pump would be strong enough for two kritter keepers?