In the sentence:
Eines gleich vorweg: Wir sind kein Speisepilz-Verein!
Why isn't it:
Ein gleich vorweg: …
What’s the reason to use genitive here? Is there any grammar that I don’t know? Can you explain it?
Here eines is not genitive and not an article. Instead "eines" is a pronoun, and its neuter form is eines.
You can translate this sentence as
One thing right from the start ...
Here, ein is not the article, it is the pronoun, and eines is neuter nominative or accusative. It means one thing is this context.
As other answers have already pointed out: Eines in your sentence is not in the genitive case, it is neuter and nominative. The grammatical role of this word can bear some highlighting.
While the eines in the sentence looks like the eines in, for example, eines Tages which is indeed the indirect article in masculine genitive singular, your example does not feature an article. Instead, it is a demonstrative pronoun, i.e. a pronoun that completely replaces the noun phrase such as the English this. (Note how this can also take two different roles: this thing is white versus this is white; the second usage is the one I am talking about.)
When a demonstrative pronoun is used, we typically need to ask what it is supposed to replace so that we can adjust genus, number and case accordingly. However, the original example does not contain (and will not contain, even if the excerpt is expanded) a direct reference so we have to assume one. The main choices we might consider for this type of construction are a person of some sort, an animal of some sort or a thing of some sort. However, context clearly shows that it must be a (abstract) thing of some sort. For things (and typically animals), the neuter genus is used in these constructions. Note that this does not mean it is replacing a neuter word. In fact, the most likely replacement would be eine Sache—feminine.
Concerning case and number: while the bit preceeding the colon is not a full sentence, even if it were extended the only reasonable choices would be nominative singular. Why? The meaning is ‘one thing’ that is stated prior to anything else and for most reasonable verb choices the ‘one thing’ would be a full sentence’s subject (likely in a passive construction). E.g:
Eines wird gleich vorweg gesagt:
Thus, the full grammatical analysis of the sample shows us we are dealing with a demonstrative pronoun, neuter, singular, nominative.
Eines is not the only demonstrative pronoun and not even the most common. Other choices are das and dieses. So why is eines used here? It is, and this may sound weird, an indefinite demonstrative pronoun, akin to the difference between an indefinite and a definite article. The word it is replacing is so unspecified, we don’t even know what it is replacing. It’s merely the most arbitrary ‘one thing’. That does not mean the other choices would be wrong. However, going from eines via das to dieses will, with each step, add more determination and emphasis to the thing that is being mentioned. Going from the small excerpt however, I assume the first thing they’re saying to be a little tongue in cheek. This means that overly emphasising it would make it sound slightly weird or too serious and thus the most natural choice is the indefinite pronoun.
There is no genitive in this. "Eines gleich vorweg" is an ellipsis for "Eines sei gleich vorweg gesagt" ("One thing said in advance"). "Eines" meaning "one [thing]", not "a".