Based on what you are looking for I would suggest hiking part of the Cape Wrath trail, some of the most rugged scenery in Scotland and definitely not touristy. You would need very good navigation skills and camping equipment (and preferably join the mountain bothies association, they have several very good ones along this trail, can be a lifesaver in bad weather). Only do this if you have good hiking/navigating experience and know how to safely ford a river.
The main problem you will face is that you only have 5 days, and most sections require longer. It would take some research to see if you can easily pick up the trail somewhere (Ullapool being a good option) and hike part of it while having a transport option out.
An easier option is doing part of the West Highland way, you can pick up the trail at Inversnaid (ferry from Tarbet). This gives you the nicest sections of the trail (lots of mountain scenery) + the bonus of a nice boat trip over Loch Lomond . It's very doable in 5 days if you are strong hikers and more importantly it is all public transport accessible. It will be much busier though, as in more people on the trail but still fairly remote hiking with great scenery
As for hitchhiking, I'm a girl and mostly travel on my own (I love the outdoors, most of my friends are not so keen). I hitchhike on Scottish islands without hesitation, on the main land only in remote areas. Basically, if there is little public transport in the area, people tend to be used to picking up hikers and dropping them of at the nearest transport hub (also known as bus stops). It's not something to cover long distances, but I use it mostly to get to/from trails and I've never had any trouble. Do exercise the normal caution of course, and try not to be completely wet and muddy by the time you need a ride!
