Learn Italian: Le stiliste hanno creato vestiti molto belli. - The fashion designers created very beautiful clothes.

Yes, but words ending in -ista are special. In the singular, they are the same for both men and women (lo stilista, la stilista). However, in the plural, they split into regular masculine and feminine endings: -isti for men and -iste for women. Le stiliste specifically means a group of female fashion designers.

How would this sentence change if the designers were all men, or a mixed group?

You would use the masculine plural form: Gli stilisti hanno creato vestiti molto belli. Notice that the article changes to gli because stilisti is a masculine plural noun starting with an 's' followed by a consonant.

Why doesn't the past participle creato change to create to match the feminine plural subject Le stiliste?

Because the helping verb here is avere (hanno). In the passato prossimo (past tense), the past participle only changes its ending to match the subject if the helping verb is essere. Since creare takes avere, the participle is locked in its default -o form.

Since vestiti and belli are both plural, why doesn't molto change to molti?