..i have 5 females and 2 males in a heavily planted 10 gallon tank, its a breeding machine with a 50% water change a week and a 25gallon HOB filter....so with 50% water changes on a 5.5 gallon i would say youll be fine with 3...females can get huge..i have some pushing the 3 inch mark...i would recommend 3 males...they stay small.
i use a 5.5 gallon as a guppy grow out tank to get the fry to 1/4-1/2 inch..i put about 10 in at a time but its run off a 10 gallon air pump and enough sponge filter for a 50 gallon tank with 30% water changes a week....
so 3 males with adequate filtration and water changes would be perfect...maybe a few plants like java fern or an anubis as well..both are low light no maintenance plants, that will help your tank stay helthy.