Some cylinder basics/gas to give you some insight.

The critical temp of CO2 (i.e. the temperature above which it can't be pressurised into a liquid) is around 31C, the critical pressure is ~74 bar/7400 kPa (the pressure required to turn it into a liquid at it's critical temperature)

If your ambient temp is below 31C the CO2 will by and large be in liquid form (which has a mass, but doesn't exert and pressure in the cylinder). The CO2 vapour/gas above the liquid is what exerts the pressure in the tank and gives you a reading on you regulator. The pressure above the liquid always remains the same while ever there is liquid in the cylinder (it will change with temperature), and doesn't start to fall until the liquid is all gone and all that is remaining is the residual gas. Once your pressure starts to fall you are literally running on fumes and you won't have much CO2 left (how long you'll get out of it depends on how many kegs you're running, your serving pressure, how many beers you pull or if there are any leaks).

From some basic maths (P1V1 = P2V2 with a little bit or rearrangement and using kPa for for ease), assuming your 2.6kg cylinder is 2.6L volume at 200 PSI (1379 kPa) and at atmospheric pressure of 101kPa you're volume (V2) will = P1xV1/P2 = 2.6x1379/101= 35.5L of gas left, which isn't much. There are obviously a few assumptions in here (i.e. your regulator is accurate, you live at sea level etc).

Long story short, if it were me, I'd refill the cylinder before the party, take the big one (or wear the extra cost of gas using the soda stream attachment). You definitely don't want your gas running out mid party. That would be a crime against humanity.

JD