No, you don't need to buy a separate premium for Rafiki.

The InLeo premium gives you more access to Rafiki's features. If you use Rafiki for deep features like image generation, GIFs or videos generation, (and probably the voice-to-text feature), depending on how often you create them, you may exhaust the credits allocated to premium subscribers, so it's either you wait for some days (that should be about 7 days) to recharge your credits or you purchase more credits.

That's the information I got following Khal and InLeo AMAs.