Welcome to the forum!

I normally walk alone , but would be less confident in another country, agreed! Nowhere, even your own home, is 100% safe from all possible injury or loss, but walking in Scotland on a waymarked route is much safer than, for example, wandering about a large city almost anywhere in the world.

Depending on the route and which day you set out, you may find yourself alone a lot, but if you prefer to join up with anybody who keeps roughly the same pace you are likely to be welcomed. Walkers over here are usually friendly, and you would have to be very unlucky, or foolish, or both, to run into danger. Just take normal traveller precautions, like keeping your valuables about your person and not left in a rucksack

. If you were planning solo mountain ascents in doubtful weather, going alone might sound foolhardy unless you can use a map and compass, but on a waymarked route it really shouldn't be dangerous.
Both routes you mention are waymarked and if you are an experienced walker and using a baggage service, you could enjoy either of them over five days. I helped to devise the
Rob Roy Way, so I have a soft spot for it. However, the
Great Glen Way is better waymarked, has less road-walking and is less challenging to fit into five days, also having a good public transport setup with frequent buses linking Fort William to Inverness. (The Rob Roy Way has no national trail status and no public investment.) Both are scenic, and have a lot of general interest, please see our guidebooks if you haven't already. Or follow the links above to look at the website galleries for each: this should give you a rough idea of the scenery.
Whatever you decide, good luck and let us know how it goes, please!