Grammar Question

I wouldn't use a comma at all. The first looks wrong and the second looks excessive/awkward. I don't have the rule for you but I'm 99% sure no comma is necessary. If 'but equal' needs to be set off, dashes would be more appropriate IMO.

I'm generally an excessive person, so I would use the two commas. I'd say either go with that or none at all, just using one seems odd.

It's #2. If you use only one comma, you're indicating that the item after the comma is a (somewhat) independent entity (clause or phrase), which it is not. However, in this case, you can also do without the commas entirely since this is a short and simple sentence. If any part of the sentence were more complicated, the commas would be helpful to identify that you are using contrasting adjectives to describe the same noun.

Dashes, which are stronger than commas, would be overkill here. Paired em-dashes are also usually used for clauses or phrases, not single words with a conjunction.

You can use the two commas if you really want to impose a "pause" there, but I would think that would be an exceptional case. If "but equal" is very important to the point you're trying to make, go for it, otherwise don't.

And yes using commas to create a pause for emphasis is a "rule" of grammar. At least I think it is.

By the way, "separate but equal" has connotations suggesting a nasty pretext, something you might not be intending.

There's no official ruling. You can use the commas if you want the reader to pause to create emphasis, or you can leave them out. It's up to you.

How about this for a rule: If you use a coordinating conjunction (like "but") to join independent clauses, you need a comma, although it can be eliminated if both clauses are very short. If you use the conjunction to join phrases or words, you do not need a comma.

I love wasting time on Styleforum, but it's bad for my productivity. (2 clauses)
Women are hot but men are not. (2 short clauses)
I spend a lot of time on Styleforum but even more on pornsites. (2 phrases)
Talking about grammar on Styleforum is interesting but addictive. (2 words)
Styleform is an interesting but addictive website. (again, 2 words)

If you don't need a comma according to some specific rule, don't use one unless it's absolutely necessary to make your meaning clear. There's no real chance for misunderstanding here, so don't use any.

Well, what exactly are you looking for?

This is my rule of thumb as well^. I spend a fair amount of time editing the work of other lawyers. Excessive commaization is one of my pet peeves. I often suggest a variant of Idler's rule -- "if you don't know why you put a comma there, get rid of it." I think the prevalence of unncecessary commas is in part attributable to the number of people who compose and type simultaneously (and do not go back and edit scrupulously). When people pause while composing a sentence in order to consider what they want to say (type) next, they often insert a comma reflexively.

There are rules (or at least guidelines) on when you must use a comma. None of those rules apply here. You're not separating independent clauses. You're not setting forth a list or series with three or more terms. You're not enclosing a parenthetical phrase.

That's actually not quite true. There are far more punctuation guidelines than most people would ever realize. Often, punctuation is used to indicate changes in intonation (which in turn often indicates something like mood, point of view, or grammatical operations). Here it is unnecessary because of the brevity of the abbreviated clause that's being contrasted with the conjunction but; however, if the structure were fundamentally the same but longer, commas would be helpful (and maybe even needed) in order to preserve the fact that there is, in fact, a change in prosody at each point where the commas occur.

Here's another example:

(a.) My sister who lives in Texas is visiting me tomorrow.

(a.) My sister, who lives in Texas, is visiting me tomorrow.

When spoken, the prosody of each sentence is different, because each sentence indicates something different about the word sister. In (a.), I have more than one sister, so this is a restrictive relative clause; in (b.), I have only one sister, so this is a nonrestrictive relative clause (I just tell my students it's "extra information"). If someone gave us these words and asked us if we needed commas or not, we couldn't make a determination, despite the fact that there is a relative clause that COULD be enclosed. We would need to know more about the sister or hear the sentence spoken in order to determine the "rule" required for commas here.