Inventory and sub category for pub/bar

Guys i run and pub i do love this programme but there's somethings i need to do to help with the inventory and categories.

The Inventory i have pints and half pints but i need this to come off the same stock as the  pints IE barrels of carling how can i link the 2 together so it off the same stock without going into a minus on the halves, With the spirits i offer a single at 25ml and a double at 50ml how can this come of the same stock, it there a way of putting a button in for say Vodka and the staff have to choose  25ml or 5oml button i tired this as a comment but it dont change the price if i can do it this way it will save having 2 buttons for vodka and one going to minus stock

I would like the same for all barrels i have once again 2 tabs pints and half is there any way of doing this the same as i would like with the vodka IE click carling and choose half or pint with the correct pricing to go with the choice