I would not put OR keep all the males together with one female. Soon your female will be so darn stressed and chased around for breeding, they will wear her out.
Not only that once the breeding shananagans start, you will evenually have very stressed out males as well , competing for HER.
Fights or alot of chasing will be going on in your 20 gallon tank, which even though it seems large, will not be if fights break out.
Tons of head bobing and push-ups will be noticed by them.
The stressed males may not get to eat as much, there will always be a more dominant male, so your others may not grow as large or be as strong.
It's probably a better idea to either cut down your male population, find suitable homes, or seperate them. Does not matter if previous owner kept them like this or not.
Better with one , male and 2- 3 females. They do live in groups, BUT in the wild there is sooooooooooooo much more room.
Not like a 20 gallon, ya know
I don't know if you have had any of them breed yet, but I just have the one pair, and let me tell you, shes always pregnant,,,
ALWAYS, and they have about 10 or more live kids, that EAT like freakin machines (tiny crickets) and then you have to find homes for them all.
** And by the time you do, and get a break from feeding these lil cuties, MOM is pregnant again, ehehhee LOL
So be careful, have plenty of space for youngsters, they have to be seperated from parents 
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PHEve / Eve
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