What product do I use to make small variable shaped moulds for resin casting? Is latex appropriate?

latex can be used. (it will deteriorate over time) but we would recommend silicone for a more substantial mould that would last longer. it depends on your budget really and how many casts you require.

http://www.fredaldous.co.uk/shop-online/zhermack-silicone-rubber.htm

one thing I would recommend though if you are casting with silicone is to use lego bricks to make casting boxes (see attached picture) saves you lots of time.