Hi again, Lizzy,
Daylight hours will be shorter than they would be later in the year, but there's still a lot to see and do. You'll have to take in to consideration that a few of the sites that you may want to visit will not be open yet, and some B&Bs may not be open until later in the year as well.
You could encounter bad weather, or you could encounter good weather. This past March was particularly bad, with one storm after another. The good news is that it doesn't happen two years in a row, so you may be in luck. I was in Aberdeen one March when they had their worst snowfall in 50 years. The following year, they had temperatures in the 60s. Go figure!
Best thing to do is dress warmly. A waterproof jacket with a removable liner would be helpful. If it gets really cold, you could always pick up an inexpensive sweater from one of the excellent charity shops in Inverness. Or from one of the outdoor shops (Tiso or Nevisport, for example).
You could try the NC500. The mountains of the west coast should be snow-capped in March, and would be particularly beautiful. If the weather is bad, you can always cut across inland back to Inverness. Skye is not too far away from Inverness, but you should spend a couple of nights on the island to give it a fair chance. Hotel/B&B prices should be really good in March. You can get rooms in many of the chain hotels for about 35 pounds a night. B&B prices remain fairly constant throughout the spring.
Both Glasgow and Edinburgh are only about four to five hours drive from Inverness. A stop in Stirling would give you the opportunity to visit the castle, and Stirling is only about an hour from both cities. You'd be taking the A9 most of the way south, so you'd be going through Aviemore (skiing area) and Pitlochry.
There is so much to see both in Edinburgh and Glasgow. Best thing to do is to get the Lonely Planet guide to Scotland, which will give you loads of information. The Steves' guide isn't bad, either, although it leaves out a lot of places which should be on visitors' itineraries.
At this point, since you have about 10 months to plan your holiday, the best thing to do would be to arm yourselves with as much information as you can, then go through it and narrow it down to what you'd like to see during your 13 days. Check out the various websites (Visit Scotland, Walk Highland, etc.), and decide for yourselves what you would want to see. Keep checking this website as well, for tips on where to go and what to see. Figure what your interests are (castles, hiking, cozy pubs, cities, museums, music, etc.), and plan accordingly.
When you get there, have a wonderful time!
p.s.: There's great music most nights at The Gellions, a pub on Bridge Street in Inverness, right across from the tourist information office.