On an ALR, you shuold contact a tickete office and try to book your seat/berth but, because you've got all week, be as flexible as possible. The sleeper tends to be quietest midweek, apparently, so that's probably your best bet.
The sleeper is priced in two portions: ticket and accommodation. The ticket is irrelevent to you as you hold an ALR ticket, so you simply need to pay the accommodation supplement.
National Rail Enquiries said:
Sleeper Berths are:
Single berth cabin (First Class travel ticket required) £39·00 per berth
Double berth cabin (Standard Class only) adult £34·00 per berth, child £17.00 per berth
Caledonian Solo*—(exclusive use of a twin berth cabin) £49·00 one way
*Valid with Standard Open/Saver/Business Saver/Weekender returns only
No railcard discounts. I believe that your caledonian solo fare is also valid with your ALR. It's not a bad price for a travelling hotel, but if you were travelling on a "normal" ticket then booking your supplement, it is a lot!
Apparently, sharing cabins isn't very common, particularly midweek, so paying the supplement may not be necessary unless you absoloutely do not want to share with a stranger (I'd be in that position myself).
If you want a seat, I'm not entirely sure if you have to pay a supplement for that.
All the info I've given relates to the caledonian sleepers to Scotland; I'd imagine the "Night Riviera" would be similar.
PS. When travelling seated sleeper to Fort William, you have to alight from the train and move into another carriage at EDB apparently, so I would not recommend that. However, you can arrive in Glasgow earlier by using "The Deerstalker" and changing onto a local class 156 service at Westerton.