[xtrslots]

Is ṣāḥib the masculine singular and ṣāḥabah the masculine plural? Ṣāḥibah the feminine singular and ṣawāḥib/ṣawāḥibat the feminine plural?

Sahabi = singular, male
Sahabiyyah = singular, female

Sahaabah = plural, male (or males and females)
Sahaabiyyaat = plural, females

Plurals often are not regular, so you just have to memorize them. . .

was-salam