Hi Dave-H

To answer your string gauge question immediately, no, or more accurately it shouldn't. String spacing can. See paragraph 5 below.

What size are your hands? Glove size is a reasonable indicator.

Edit: I just searched to check which Pacifica you have. Found it. Post edited in consideration. Given its age & (?) origin, have you had it checked for fretboard level, fret & nut condition, and importantly set up? i.e. action height and radius alignment. Not famil with the C2 suffix, but EG112 was the Alder bodied predecessor to the 012. Being so old, I'd recommend fitting a new fresh set of 9s unless it has been done recently TYK.

New Pacificas come OOTB fitted with 9s. Unless your hands are truly petite, you don't want lighter. If yours was second hand it might be fitted with 10s or questionably set up. Any or a combination of 10s, [u]high action[/u] and or a damaged or poorly cut nut might be contributant external factors.

The nut width on your nylon string acoustic is as narrow as a Pacifica? Unusual. What about the string spacing? Contemporary [u]Pacifica string spacing is unusually narrow at 10.5mm as is its nut at just 41mm.[/u] There are exceptions, but [u]most[/u] acoustics run 11mm or wider IME, with nut of 43mm or wider.

Re first form C chord. Open C is an easier chord, but can seem a bit of a stretch initially. Be sure you're not inadvertently applying excessive pressure to your forefinger during the fretting of the chord flattening out your tip unnecessarily or causing interference inadvertantly through placement angle because of the stretch. Applying more pressure than necessary is a common 'sin' starting out. Mea Culpa.

Pay particular attention to its overall fingering to determine why the forefinger fingered C note in particular isn't sounding cleanly if that's what is occurring.