My question is whether you use the plural or singular form — or either. Is there perhaps also a different usage in the US and the UK?
In the US company I work for in the UK, it's called collateral (assuming you are talking about the printed material which supports a sales force).
In fact I would say — if it's used as a noun — it's an uncountable noun and always singular, even if you have several different types of collateral from different companies or used in different situations. ("The companies' collateral caters for their different spheres of operation.")
The word is probably properly an adjective meaning "accompanying" (collateral material accompanies a sales pitch); but it's become a noun in its own right, at least in business-speak. Due diligence has suffered in much the same way.