Hi, last year I spent 3+ weeks in Scotland and drove everyplace except Glasgow and Edinburgh.
I drove/ferried from Glasgow to Arran, spent 3 nights there, then drove/ferried to Mull, spent 4 nights there, and then drove/ferried to Skye for another 4 nights. All of those places have single track roads that are windy and somewhat mountainous.
Arran is smaller and a bit greener, and therefore you're not driving on such long roads as you will find on Mull and Skye. But because it's smaller, you will not see the sights you would on Mull or Skye. But there were single track roads in the mountains there.
Mull's roads were certainly doable, although again single track. Some of them went up into low mountains, but nothing that was too severe. And I have a very strong fear of heights so if I can drive on Mull, anyone can. There was one place that I backed away from and that was the drive to Calgary from Tobermory, which takes you over a number of switch-back roads in the mountains. I took one look and turned around. If you do go to Mull and want to drive to Fionnphort and take the ferry to Iona, make sure you take the southern route (through Craignure) and not the northern route, which is much scarier (I've heard).
Skye's roads were about the same, although Skye actually had more double-track roads in many places than Mull did. For example, the road from Armadale to Portree is like a highway. Two lanes but moves pretty quickly unless you're stuck behind a camper van. The road from Portree to Dunvegan and Edinbane is also pretty easy to drive, as is the road heading north through Uig. But of course there are a number of single-track roads there, and you have to be careful.
I will tell you that Lewis and Harris was probably the easiest driving I had on the Isles, but that's pretty far north for you. I have not been to Islay and Brute so can't help you there. But I would suggest going to YouTube and searching for videos that discuss driving in the places you are interested in. There are a lot out there. You can also use Google Street View to get an idea of what the roads are like.
But really after a while you get used it. I went back this year to England and drove around for over 4 weeks, and while Cornwall's single track roads were pretty difficult (in part because it rained so much while I was there), overall everything went pretty smoothly. By the way, getting the car on the ferry is really easy. After the first ferry, you will become a pro at it. :-)