Learn German: Du kannst welche aus dem alten Vorrat nehmen, wenn du häkeln willst. - You can take some from the old supply if you want to crochet.

In this lesson's context, welche acts as an indefinite pronoun replacing a previously mentioned noun. While einige usually refers to a countable plural (a few) and etwas refers to an uncountable amount (some), welche is a highly versatile conversational word that stands in perfectly for both plural nouns and uncountable mass nouns.

Since welche replaces a noun, what gender is it taking here?

It is in the accusative case as the direct object of nehmen. When used as a pronoun for mass nouns, its ending matches the gender of the noun it replaces. Because it is welche here, the unmentioned noun must be feminine (like die Wolle, the wool) or plural. If the crafters were talking about a masculine noun like der Faden (the thread), the pronoun would be welchen.

Why does the adjective have an -en ending in aus dem alten Vorrat?

The preposition aus always forces the dative case. Der Vorrat (the supply) is masculine, so its dative article is dem. Because the article dem already shows the strong case signal (the 'm'), the following adjective receives the weak ending -en.

Could I change the word order and put welche later in the sentence?