Enabling c_t and longs_t without enabling s_t ligatures

@tohecz: Thomas's Q is closely related to the questions I referred to, but it seems he's suffering from a problem that's not addressed in these.

I tried to use the selnolig package to selective disable the s_t ligature (\nolig{st}{s|t}) but that also stops the longst ligature from being used.

If I understand correctly, you're expecting selnolig to differentiate between an s and ſ. You're not providing an MWE, but I'm assuming that in your case selnolig just can't differentiate between the two because you don't -- I guess your text only has s and s, so there's nothing selnolig could do for you.

Educated guess: entering your ss correctly is going to produce decent results.

• selnolig disabled. Minion Pro, all available ſ_ ligatures as well as c_t and (inappropriate?) s_t are present.

• selnolig enabled. As above, only s_t is gone.

code:

\documentclass{scrartcl}
\usepackage{fontspec,selnolig}
\setmainfont[Ligatures=Rare]{Minion Pro}
\nolig{st}{s|t}
\begin{document}
The ſide attraction for moſt ſpectators was the mustard
\end{document}

How about the ſ_t? -- I don't have Hoefler Text, and Minion Pro doesn't have an ſ_t (technically, it does, it's just indistinguishable from unligated ſ t). But, for example, EB Garamond does, and it's not affected by \nolig{st}{s|t} (q.e.d.).

PS: if you're planning to reproduce early 19th century style, Hoefler Text (as well as Minion) will be a bad choice. It's off by at least two centuries, as is obvious from the two Encyclopædia Britannia pages (from the second one even more drastically so than the first). Have a look at a Hoefler's Didot instead, maybe Old Standard or -- inexpensive and most appropriate -- Miller from Font Bureau.

update

Looking at your own answer and your insisting on the idea that a mere Contextuals=Inner is going to suffice to produce correct results -- what needs to be stressed is that correct ſ usage is not as simple as »replace all non-final s by ſ«. I suggest you have a look at how the ſ was handled in the period/the culture whose style you're trying to reproduce. In what contexts was it used? When was a s used instead? Think compound-word word boundaries. What ſ_ ligatures were available? Think, among others, ſ_s. In what contexts were which ſ_ ligatures appropriate/inappropriate? Again, think compound-word word boundaries. What was considered a compound word in the first place? etc.

Further reading: http://babelstone.blogspot.ca/2006/06/rules-for-long-s.html