For a plumber this is probably a very simple question but I am generally used to high pressure hydraulics where fittings have to match exactly. Last week I had a plumber round to disconnect an old tap and put some isolating valves on the water supply. the valves he put on are 15mm type with olives, I am now looking at the new mixer tap I want to fit and the flexible pipes that came with it fit perfectly to the ball valves, all I have to do is take of the nut he left on the ball valve pull out the olive and screw on the nut on the end of the flexible. But the flexible pipe has a rubber seal at the end and obviously doesn't match the sealing profile. Is it okay to put a soft seal type connection on to the isolating valve or should the plumber have fitted a different type of isloating valve.