That's right - with the added caveat of
"true 15mm and 17-18mm" but also anything inbetween (and a bit bigger). If minis call themselves 18mm (like Eureka's fantasy range) you can bet that they'll be pretty big.
I mostly do sci-fi, so don't worry about scale massively (I'd only really worry if I was multi-basing and putting minis on the same stand). However:
Copplestone barbarians are meant to be big, tall and muscley. They're scale is pretty true, a touch big maybe, but they look big bigger than a lot of 15mm, particularly historical. It's right, but can look worng... if that makes sense? They're absolutely brilliant minis though, so if in doubt avoid other minis that don't fit and use the Copplestone.
Most of the Ral Partha daemonworld range will fit really well.
Essex are relatively small, not tiny - I use Essex civilians without a worry. I haven't seen the Khurasan skellies in the flesh (they look really nice in the pics on the site) but I'd be surprised if they didn't match the Copplestone well based on other Khurasan bits I've got.
I find Peter Pig too small. They're true 15mm.
Doesn't bother me, but it sounds like you might find Blue Moon too big (nont sure they do fantasy anyway actually).
Donnington New Era are a pretty good size (smaller than Copplestone, a touch bigger than Essex but of a very similar style so fit well). Older Donnington range is pretty small.
I'm a big, big fan of Forged in Battle - they're slightly taller (~17mmish) but quite delicate so don't look huge.
No easy answer I'm afraid but if scale really concerns you it's probably best to hover around the Copplestone-Daemonworld -Khurasan zone.
Lastly, don't be afraid of 'shimming' minis up a bit - the eye is a funny old beast and if the heads are at the same height you won't really notice, even if the shorter chap is stood on a little mound.