If you're only planning to use
Jessie, then surely it doesn't matter much if you give her a "full" name or not. What would matter, to me anyway, is that giving a child just a nicknamey name means they have fewer choices later on, when it'll be out of your hands.
I was at school with a
Jessie Mary; she had Scottish ancestry, and it seems that
Jessie can be or once could be a nn for
Janet. That would be a stretch! But
Jessamine nn
Jessie would make perfect sense and give her
Jessa,
Mimi,
Minnie etc as options - and she might not want to change, but if a porn star or a serial killer named
Jessie ever lives in the same town, more wiggle room rather than less could be very welcome.
The Jessicas I know have all been
Jessica or
Jess. Not a
Jessie among them!