Learn German: Ehe wir anfangen, holen wir allerlei Fläschchen. - Before we begin, we fetch all sorts of little bottles.

It doesn't decline at all! Words ending in the suffix -erlei (like allerlei, zweierlei, and vielerlei) are invariant adjectives. This means they never change their endings to match the gender, case, or number of the noun they describe.

Is Fläschchen singular or plural here, and how can we tell?

It is plural here. Nouns with the diminutive suffix -chen are always neuter, and their plural form is identical to their singular form. We know it refers to multiple little bottles because it is paired with allerlei (all sorts of), which logically implies a variety of items.

Why is the word order holen wir instead of wir holen?

The subordinate clause Ehe wir anfangen takes up the entire first position of the overall sentence. In German, the conjugated verb of the main clause must always be in the second position, so holen comes immediately after the comma, followed by the subject wir.

What is the difference between ehe and bevor?

They mean exactly the same thing and act as subordinate conjunctions. Bevor is generally more common in everyday speech, while ehe can sound just a little bit more formal or literary, but they are completely interchangeable.