Yeah...you'll hear 1 fish per gallon, or one inch of fish per gallon, at lot, almost entirely from people who don't know what they are talking about, usually in petshops. Here's an extreme example of why it wouldn't work - imagine ten angelfish in a ten gallon, or a ten-inch oscar. Conversely, one little tetra would be very sad in a one gallon. That rule doesn't take into account a whole bunch of things, including territorialness/aggressiveness, bioload and schooling needs.
Fiery is very cute! Does she have a heater in her little bowl? :)