That sign for candy - twisting your index finger on your cheek - is prevalant in BritishSL. And Mangetout is in Britain, which would explain your confusion Handy.
[aside]When I was three my mum was asked to do some demonstration poses for a basic sign language book that was being produced. She took me along to the photoshoot and the cameraman must’ve liked me or something because he got me to do lots of signs and I am in the book too doing the signs for “monkey” and “ice cream” and “spider”. It’s out of print now, but I hold my copy dear. I look so cute! [/aside]
As for Deaf poetry, there is indeed such a thing, but I’m not sure if you’d call anything within it “rhyming”. I would think more in terms of visual symmetry - one sign maybe mirroring another or complementing it. It’s more using the unique way sign language can convey emotions to effect. Yes, “symmetry” is the word I would use, rather than “rhyme”.
The most famous Deaf poet I can think of (and by that I mean a poet who uses sign language as her medium, rather than a poet who happens to be deaf) is the late Dorothy Miles. Here is some of her work, but having it written down kind of takes the life out of it. I’m searching for a video online but I can’t seem to find any. A Google search on her name yields much of her poetry and discussions on the nature of deaf poetry.
If you’re really interested you could take a look at some of the articles here.