we are planning to book nothing in advance
If you have a tent and a vehicle, you have no problem, you'll always be able to sort yourself out with that. The weekend you arrive is the end weekend of a school holiday in Britain, so accommodation will be very busy that weekend. After that you'll find things fairly quiet, as it is not holiday season. So most days you will find B&Bs and hostels if you want them.
I definitely want to do Skye. Also the Orkneys are high on my priority list. Any tips for in between
Skye has a very high profile for tourists, but actually many other less famous parts of Scotland are just as wonderful, with the benefit of lower tourist pressure. Though if you are into difficult walking along exposed jagged ridges, then the Skye Cuillin is unique.
Actually there is a very special part of Scotland part way between the ports for Orkney and for Skye. You will find the NW coast of Scotland to the north of of Kyle of Lochalsh very special, especially Torridon and the region around Lochinver. Castles and distilleries, your guidebook will advise.