OK,
thanks for those replies…
I guess my REAL question is this:
If I realise and dont mind that my SD footage will look less sharp than my HD footage, I gather that it is perfectly OK to include it in my HD timeline, and it will all be playable on HD tv, but the SD sections will just be lower resolution. But there will be no compatibility problems when watching it on HD TV? Its old footage, so i dont think it will matter if it looks blurry and low pixel count.