I use these pegs to tune hurdy gurdy sympathetic strings. They?re compact, stable, look good, well built and are cheap. An inexpensive fitting key is easily available.
I would recommend drilling test holes between 5mm and 5.5mm in a piece of wood and try screwing a peg in to make sure the diameter is correct ? sometimes drills can be a slightly different size to what they say they are!
The product page would benefit from more detailed technical specifications about the product ? how long the threaded part is, where the hole is, the diameter of the hole, the size of the squared part. A technical drawing would be perfect.