Garibaldi hardly would fight the Piedmontese unless caving fully to Mazzini's pleas. Aside from the fact the Bourbonic remnants were standing on the Volturno, an army from the North was arriving and Garibaldi was aware of the difficulties he would face. Besides his aim was the Italian unity, not the regime in vigour. While being of republican sentiment (but not so strenously as certain post WWII propaganda let believe), he was still fine with a monarchic Italy as long to be democratic. Besides he refused certain honours but others he accepted after the unification.
To have a shot, at least two conditions should be favorable 1) Republican riots exploding in the North, almost impossible because Mazzini's radical republicans were very marginal and so the Partito d'Azione 2) France staying neutral, impossible because Nappy III would have not stand for it.