Definitely shake the 250B resin every time before using since the wax tends to sink to the bottom. You might also want to make sure the board is free of dust and debris before hotcoating. That will cause pinholes and bubbles.
Yes, you can add styrene, wax and MEKP to 250B for glossing. Surfacing wax has styrene in it so you can cut back on the additional styrene a bit so it's not too thin.