To be blunt you can pretty much do what you like in terms of planting and style, some of the most interesting plant combitions are a complete juxtaposition. For example what you may quite often see is box forms and hard lines with loose perrenial planting within or around them, I have seen 18th century roses in the midst of only very newly released cultivars of certain perrenials.
I believe the best way of approaching a garden in terms of plants is to have beneficial relations between each plant and its situation - where the information is accessible and the overall design allows it try and place a plant as close to its natural and native situation as possible. So for example with an echinacea give it great big baking space and give its feet lose dry ground. If you liked the look of a Helebore give it a nice cool spot then add another shrub layer with a Daphne and then another layer with an arching Hazel ( I only passed some today on the motorway bursting with dusty yellow catkins) to provide the final dappling of shade.
There are endless things to do and you are only governed by the light, the ground and what is already in situ. There will be plants that need help in terms of soil imporvment and some that will be perfectly evolved to suit the exact conditions wihin your garden. When it comes to appearance you will probably be the best judge but what I would usually lean towards is less is more, green is the most usable colour there is and there is never to much of it. While garden rooms and juxtapositions are interesting, an overlying story will allow more room for change in the future and tweaking as you spend more and more time there. A separation or divide in the garden can eventually cause stress in keeping boxes ticked and growth in check. Also buy herbacious perrenials young - don't spend money on larger root or pot sizes. Try to get 1L pots sizes, they will grow quick and move fast, some 2m in a year, but if you want a certain flower or a tree to do a certain thing you may have to buy older and more expensive specimens... Also try out seed mixes, annuals and native flowers, packets are cheap enough and they are great to watch and smooth gaps out. When planting plant in blocks (not a block shape...unless you want that) for example in threes or fives to have a more lasting affect like those drawings that are just dots up close but form a picture the further you step back planting is the exact opposite you will lose a flower very quickly in among its neighbours so plant more of the same to unite the affect you're after.
Hope that helps.