The differences between series regular and recurring guest-star are minimal if we're talking about the number of episodes an actor is going to be in.
In fact a series regular can appear in less episodes than a special guest star. When Mark Sheppard was bumped to series regular, he said the main difference was that his contract now stipulated he couldn't appear on any other show during the season.
I imagine that for an actor like Pellegrino who gets role offers from fucking everywhere, it makes sense to put him as series regular to secure his appearance in several episodes in a row.
You're partly correct on this. Being a series regular on Supernatural doesn't mean you're in every episode but it does mean -- at least so far in its 12-season history -- that you're in at least half of the season.
Misha and Mark have "half season regular." Here's the episode count for Misha when he was as series regular:
5 - 14 episodes
6 - 12 episodes
9 - 11 episodes
10 - 13 episodes
11 - 13 episodes
He was in 5 episodes in season 7 and 9 episodes in season 8.
For Mark:
10 - 12 episodes
11 - 11 episodes
Before that, he was in 3, 8, 5, 9 and 9 episodes from seasons 5-9
Katie Cassidy's Ruby and Lauren Cohan's Bela were only in 6 episodes each during season 3. But that season was cut short from 22 episodes to 16 due to the writer's strike. Otherwise, they likely would have hit 11 episodes each.
So with that being said, I would not doubt at all if they have plans for Pellegrino to be a series regular from the start of season 13 and for him to be in at least half of the season.