Can I use a pint/ 500ml whipped cream canister for small volumes?
You can (to some extent) fill it under the max line and still get good results.

The standard 500 ml whip has an internal volume of 750 ml actually. If you fill it up to the max-line, you’ll have 250 ml left for the gas in the cannister.

Ideal Gas Law states that, pressure changes inversely proportional to the volume of the container.

So... say that you’ve filled up to 250 ml, you’ll extend the volume of the gas twice; thus the pressure will drop twice.

Henry's law on the other hand, states that amount of dissolved gas is proportional to its pressure.
So, the amount of NO2 you'll get dissolved will be also halved.

50% less gas should be your lower-limit I would say. You'll probably have less volume also on your whipped cream as well. If that's something ok with you...

But this approach will signifacantly lower your yield too. As the leftovers in the cannister will be constant, not proportional to your starting amount.