Generally an item that needs multiple units is going to be an inventory type item. So you probably don't want to set a situation where your going to end up with odd-ball decimal values of stock. Nor do you probably want stock quantities with 9 digits that wont print. The 'base' quantity you set for the item will be the quantity in which the inventory tally & valuation is maintained. Using gram as the base could mean you never deal with decimal stock, but who wants millions of item count on the inventory tally - that looks ridiculous. If you use both both pound and kilo units could lead to odd decimal leftovers (buy 10 kg and use 20 lb - whats in stock .92815?) If you must mix metric & imperial then maybe pound is a good base, as the ounce is an exact decimal 0.0625 and you can use 2.20x for KG. But if you try to be exact and use 2.20462x for KG then your stock is always going to be a 5 decimal number.
In the end its a pretty arbitrary decision as to the base and alternate units you use - choose what you like to work with.
