Displacement is not really an issue. Even from the gigantic 300bar cylinders you get from BOC now, you only get ~12m^3 of gas. In a theatre that’s virtually nothing. Helium always rises, even at cryogenic temperatures (4K), so unless you have someone with their head placed against the ceiling, asphyxiation is not an issue. At the flow rates you are probably going to be looking at it’ll be the same as welding argon.
The problem that might set you back is the sheer cost of helium at the moment. There is currently a shortage, and BOC is rationing bottles of pure helium to uses with a good use case and based on previous order volumes. You’ll probably be able to find balloon gas, that’s a diluted He mixture with oxygen and CO2, and that’s what you’ll have probably have seen in most shops. A large cylinder, standard cylinder size would be £384.29 from BOC, plus the delivery and cylinder rental and ~£250 for a regulator.
At these sorts of prices, it would be worth doing a test by getting a balloon from a shop and making some bubbles first to see if the effect works.
