small gf and fancy gf need a minimum of ten gallons per fish when small. after a few months the need for more space grows as gf are very messy fish. the filtration also needs to be good to keep the tank clean.
for a 7 gallon tank you will only be able to keep some white cloud minnows. i dont know about any other type of cold water fish though so maybe there is something alternative you can get. i have white cloud mountain minnows and they are very colouful fish. you could maybe get about 7 of them in your tank and with good filtration maybe 10.