Would [name]Sadie[/name] be too far fetched? Other ideas?
I think [name]Sadie[/name] is too much of a stretch for [name]Seraphina[/name]. I’d use [name]Sera[/name], Seri, Phina, Phia, or Sia.
I’d probably use [name]Siri[/name], I love that name. 
I know a Sepy… I have no idea what her legal name is. I think it’s kind of cute though.
Another nickname option for [name]Seraphina[/name] is [name]Fifi[/name].
What about [name]Suri[/name]? [name]Raff[/name]? [name]Eri[/name]? [name]Sarah[/name]? [name]Rina[/name]?
Fi!
I love [name]Seraphina[/name] nn [name]Sera[/name]- but I also could see Serie, Phina, PhiPhi/[name]Fifi[/name], Phi ([name]Fee[/name]), or just S. A new one that nobody’s said yet is [name]Sissy[/name]. You can use [name]Sissy[/name]/[name]Cissy[/name] for most names starting with S or C 
I have to second the nn of Fi/Phi/[name]Fee[/name]/Phee.
I just love it!!! =D
I don’t think that [name]Sadie[/name] is too much of a stretch for [name]Seraphina[/name]. [name]Sadie[/name] is a nn for [name]Sarah[/name] and [name]Sera[/name] is at the beginning of [name]Seraphina[/name]. If you like it, go for it! I also like [name]Fee[/name], [name]Sera[/name], [name]Rinna[/name], Saphy, Epha (like [name]Aoife[/name]), and [name]Hera[/name] for [name]Seraphina[/name].
I agree with [name]Sheila[/name] - [name]Sadie[/name] makes sense to me, since it’s a nickname for [name]Sarah[/name]. I would probably use [name]Fina[/name], but [name]Sadie[/name] or Seri are great, too!
I though of Phina/[name]Fina[/name] as a nickname but Sia is great!
I love [name]Raff[/name]!
Whatever you call her can be her nn so if you like [name]Sadie[/name] that is fine. Another name that I love that could also be a nn for [name]Seraphina[/name] is Sepphie.
[name]Sadie[/name] is most definitely too far-fetched for [name]Seraphina[/name] in my opinion.
They’re radically different to me.
Ideas: [name]Sera[/name], Phina, Seri/[name]Siri[/name], [name]Nina[/name], [name]Ari[/name].
I love [name]Seraphina[/name] and Sepphie as a nickname. This is a serious contender for me!
I wouldn’t think of [name]Sadie[/name] as a nn for [name]Seraphina[/name] but I guess it’s not too far fetched - a friend of mine’s name is [name]Louise[/name] and her family call her [name]Lucy[/name] more than they call her [name]Louise[/name] which I find quite bizaare. [name]Sadie[/name] is a nn for [name]Sarah[/name], so [name]Sera[/name] (phina) nn [name]Sadie[/name] isn’t too outrageous, as someone pointed out earlier…
I do prefer Fi/[name]Fee[/name] though. So spunky! And still unexpected as most people would likely assume her full name was [name]Fiona[/name]
My mother is Dutch and her name is Fietje (pronounced Fee-cha), which in Holland is a nickname for Josephina (her given name). Since that’s based on only the -phina part, which that name shares with Seraphina, the same nickname could be used. Of course, Sera would be the more obvious nickname for Seraphina.
Edited for privacy.
I know one who goes by [name]Sera[/name] (sounds just like [name]Sarah[/name]) and [name]Fina[/name], which I really don’t like. Personally I don’t get the love for this name.