A single betta would be a perfect choice for this tank. You cannot have 2 bettas together as they will kill each other.
Good for you for following all the great advice you've been given. I can't believe someone at a fish store would have sold you all those fish for that little tank. Shame on them.
But again - good for you. Return the fish asap. start your fishless cycle, get a heater and a larger filter and soon you'll have a wonderful environment for a betta who will give you hours of pleasure.
30 g FW planted:corys, ABNP, blue angel, harleys, zebra danios, nerites & mystery snails
15 g FW planted: crown tail betta, neons, snails
90 g FW semi planted: Blood Parrots, severum, Jurupari, EBJD, congos, kribs, clown pleco, snails
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Fishless cycling: http://www.aquaticcommunity.com/aqua...ead.php?t=5640
Cycling with fish: http://www.aquaticcommunity.com/aqua...ad.php?t=36492