The one overall suggestion is to test your new layout for conversion. Content is good, but is not 100% needed in the amount you have on what seems to be ecommerce category pages. Focus on the conversion and having some content.
1. The new pages are only 180-300 words of text, simply because that is all that is needed to describe that category and provide some supporting information. the pages previously contained 600 words. Should I be looking to get more content on these pages?
No, I've seen categories with less content rank very well. Remember, names of products and other supporting text on the page is content as well. Don't worry about the length.
2. If i do need more content, It wont fit "above the fold" without pushing the products and sub categories below the fold, which isn't ideal. Should I be putting it there anyway or should I insert additional text below the products and below the fold or would this just be a waste.
Nope. Not unless the user needs it.
3. Keyword Structure. I have designed each page to target a selection of keywords, for example.
**a) The main widget pages targets all general "widget" terms and provides supporting **information
b) The sub-category blue widget page targets anything related and terms such as "Navy Widgets" because navy widgets are a type of blue widget etc"
Is this keyword structure over-optimised or exactly what I should be doing. I don't want to spread content to thin by being over selective in my categories
What you are doing sounds reasonable in theory. Stick with that and keep in mind that you only need to worry about specific terms like navy widgets if there is more than one navy widget and there is a high volume of demand for that. There is a lot that goes into this, but try to keep the category page focused to 1-2 terms.