I just drilled the holes out to a larger diameter to push fit them in to the stock - the drill bit I used ( can't remember size ) may have been a little on the large size so had to use a bit of araldite so they stayed in place . A gardiners brush stock is not deep enough for counter sinking to be of any use for these jets. They will protrude when bristles are splayed but have not had any issues in the last 12 months use - you would have to apply a lot of pressure in any case but just be aware of it.
Standard jet hose is not going to fit over the barbs, even warmed up in hot water I could not get them on. I had to use a short length of pole hose on them and then barbed reducers down to jet hose diameter.