It'll work fine. the 2.5 and 3gal cornies aren't as common because you only find them new, for like $140 a pop. That's 3-6 used 5 gallon cornies, so many guys just 'go big', that's all.
nothing at all wrong with your idea.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10