For a while I've had a few ideas about a less then ethical genetic program. This particular project requires power armor but I was wondering if the space marine style power armor was the only one the Ad-Mech manufactured. I know there are human characters like Tyrus that wear power armor but I was wondering if there are any examples of something a little less bulky?
Dark Heresy et al give us "light power armour"; this (http://www.games-workshop.com/gws/catalog/productDetail.jsp?catId=cat1140003&prodId=prod1140024a) is a possible representation.
There's also what the Adepta Sororitas wear, and although I have my doubts that it's actually proper power armour, Ascension also gives us Ignatus-pattern power armour (as worn by this guy (http://www.games-workshop.com/gws/catalog/productDetail.jsp?catId=cat1140003&prodId=prod1630026a)).
So the real answer to the question in the thread title is "very".
Battle Sisters are an easy example of the common production of non-Astartes armour.
Actually, although old fluff numbers would disagree with me, I think Sororitas armour might even be more common than Astartes armour - Marines are very rare and specialised individuals amongst a gamut of military forces, whereas Battle Sisters are the only standing military force the Ecclesiarchy actually has.
As far as diversity, I actually doubt power armour is mass produced anywhere - there won't be long production lines churning it out, it's made suit by suit. So while internal components will be made to conform to whichever STC based pattern, the actual ergonomics and aesthetics are presumably customisable suit by suit.