The reason I don't do first in series free is that the first books in a couple of my series sell the best. They tend to be the most over the top. Maybe my sequels aren't wild enough for readers; don't know. The couple of times one has been free during a promotion, I haven't seen a sizable sell through, which means nothing good.
I do have a free novella now that links to one of the series, but it doesn't introduce it. Maybe I should. OTOH, it could be throwing good money after bad.
I think the next move for one of those series is an omnibus edition with the novella added as a bonus, but since other authors routinely give those away free or at 99 cents, I'm not falling all over myself with eagerness to pay for a new cover, ISBN, formatting, and so on.