You are absolutely right. -wyn is masculine and -wen is feminine traditionally. Occasionally, there are a few instances that break this rule but not usually.
Caerwyn is a masculine name. J.J.Caerwyn [name]Williams[/name] was a well known Welsh scholar. It means “white fort”. Carwen is a feminine name that means “white/fair/blessed love”.
In South [name]Wales[/name] Caerwyn would by [name]KEYE[/name]'R-wyn (with a long ‘i’ sound). In [name]North[/name] [name]Wales[/name] it would be KAYR-wyn (with long ‘a’ sound).
Carwen would be KARR-wen with a long ‘ar’ sound.
This link should help:
(although for some reason the soundbite for Caerwyn doesn’t work and sends you to the Carwyn one, so that one isn’t correct. All the others are though).