Not all livebearers have gravid spots. She probably had the fry from stress. You can use the 10g for the fry and feed them crushed flakes and BBS. Put some java moss in there and you'll be set. They're not hard to take care of. Usually, if I try to keep my platy fry, they die, but if I leave them alone then they live.
Fill up the 10g halfway and put some live plants in with them. Feed them BBS at first and then start feeding them crushed flakes. Once they're about 1/4" long, fill up the 10g fully. Let them grow until they're almost an inch and ask if any places can take them. If not, see if your family can take them.
100g Native Fish Pond, 55g Goldfish/Catfish Community, 50g Plant Pond, 40gB Tetra/Cory Community, 20gHM Community, 20gH Turtle (Bender [RES]), 20gL Turtle (Kif [YBS]), 15g Reef, 5.5g Betta (Okami), 5gFBH Betta (Pron), 5gHex Betta (Twister)
2 dogs, 13 cats, 2 turtles, 1 bearded dragon and 1 crested gecko