Many livebearers can do that. I don't see why guppies would be any different. It also says so on the guppy Wikipedia. Says they can store it for up to a year
Yes, I've read several places that they can store sperm for at least 4 pregnancies from a single insemination.
I'd estimate she was no more than 0.75 inches when she gave birth the first time, that was a little over 1 month ago so a little under 4 weeks from the day I brought her home. She's defintely bigger/longer now than she was the first time she had babies.