So, I've been recently testing several endurance bikes to replace my 58cm CAAD-9 I can no longer comfortably ride. I've been riding mostly 58s with a smattering of 56s. Reviewing the geometry charts, there seems to be very little overall difference in reach (about 5mm, most of the times), with 10-20mm differences in stack and HT length. Does this imply that it would be relatively simple to fit someone who rides a 56cm bike to either a 54 or 58 with things such as stem swaps and seatpost/saddle adjustments?

I ask mostly because I ended up buying, sight unseen, a 58cm Scott Solace - I had test rode the 56cm (they didn't have anything larger in stock) and it felt OK, but after testing the 56 and 58cm models of the Domane, Defy, and Roubaix and preferring the 58cm models, I assumed (after looking at geo. charts) that the 58cm Solace would provide a better fit. Now I'm kind of worried I made the wrong decision and should've just went with the 56. I thought the 58cm Scott, with about 20mm larger HT length and Stack would be good for someone with long legs and a bad lower back, but I don't know how much difference 5mm reach and 15mm top tube would matter or if I could fix (potential) reach issues.

I'm 6'1" with a 35" cycling inseam, so I've always had trouble finding the "perfect fit" stock bike as I seem to be on either the large end of a 56 (feeling slightly cramped and with too much saddle -> bar drop) or the small end of a 58 (feeling a little too stretched out).