I know I'm late to the party but here's my 2 Rubles. You don't need a bag unless you're planning to carry something, then the bag only needs to carry what you absolutely need. I keep a small tool roll with had wipes and meds, that I put on a jersey pocket so, on my clincher wheel bikes I only need a small bag for tube and levers (and sometimes a CO2 charger. There are lots of options for a bottle and if it fits I recomend on the DT and running the pump along the ST or even the seat stay. A handle bar bag would be useful for both picking up the odd snack or something as well as an extra bottle for longer rides.
Not the best pic but the pump fits nicely in the old school along the seat stay arrangement. I'm not sure they make a smaller seat bag
This old Cannondale bag doesn't fit quite right with this Hybrid stype stem but it's very useful for errands
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk