I don't have LFSI have to be my own LFS!
I've never kept sparklings, but they look very timid to me. I doubt they would hurt the guppies -- I'd actually worry that your guppies might hurt them...
Sparklers are indeed timid fish, in my experience. They also prefer more cover than being in the open so floating plants are pretty much a must IMO.
If anyone would be the aggressor it would be the guppies for sure. They also do not have a very long life span.