>as you can ignore brake lights and horns as intermittent loads
As the thread title is "fuses", no, you can't. There is no such thing as an "intermittent load" to a fuse. When current draw exceeds the fuse's 'blow' rating, it'll blow - that's what it's there for - the fuse has no concept of whether the excessive load is a short-circuit or you tooting the horn at a pop-up peddie.
Seen it several times (luckily not actually been that stupid) on rewires of later bikes - rewirer thinks the standard fuse is too highly-rated and fits a lower-rated one; riding along with lights on, indicators on to turn, brake (with or without tooting the horn) and ... the too-low-rated fuse pops.
You're absolutely correct - if the bike's new electrics will have a main or single fuse, it must be rated high enough to transmit all individual loads at the same time.