Generally best to follow the guidelines - but sometimes it's possible to find ways to adhere fully and yet to still do the things you want to do…
Your plan to use a septic tank for black and a surface wetland filter for the grey makes environmental sense, as long as the grey water wetland filter will do
enough before the water gets into a natural habitat (such as a bog, stream or groundwater). I'd usually recommend a biggish area for that job: about 50m2 for grey water only, prior to discharge to a percolation area. There are much smaller filters, or buried systems such as Oasis Designs work, so have a look around. have a look on my website if you want to get some background on the differences between constructed wetlands and gravel reed beds. The gravel reed bed would only be about 15m2 for the grey water, but needs the gravel so it's not necessarily cheaper - depends on site conditions.