When I started the Seven Spires and included Ptolus in it, "Ptolus: City by the Spire", the big book, didn't exist and wouldn't, as far as I knew. Now, if I was to use the Seven Spires described here with the Big Book, I would use a blend of Ptolus and home-made handouts. I would discard mostly the information regarding the Empire of Tarsis and the history of Praemal while still giving the Player's Guide to the players, and replace the information with additional sheets of papers presenting the alternate timeline and regional information regarding the Seven Spires.
My use of the Seven Spires in my campaign ended last year with the PCs altering the very fabric of the multiverse. This resulted in them emerging into the Ptolus of Praemal instead of the Seven Spires described here where they lived. I can thus use Ptolus within Praemal for my campaign while explaining why, retroactively, it exist(ed) on the Seven Spires.
I have plans for the Seven Spires after the multiverse was altered by the Spellwardens (the PCs). Basically, I would replace all the mentions of IP of other publishers and authors (like Laelith, Ptolus, Manifest) by some of my own creations. This would then become a publishable setting (which wouldn't challenge these IPs, particularly).