The capacity of an angel kit is its maximum stock, so no, a gunlugger who's prepared for the inevitable can't just buy it up to 6-stock.
Can you elaborate on this? Why wouldn't the gunlugger be able to purchase more than it's capacity, or for example, buy two first aid kits and combine them for a 3-stock roll? You mean he can only carry around 2-stock "on him" right?
I guess the question should be, if fictionally, there are more medical supplies around, say the gunlugger's first aid kit, plus 2-stock of medical supplies he bought, he can use them right?