For reference, I'm using a 10w magnetic ballast for fluorescent tubes, and the mains is 220-240v 50hz.

The switch I'm using in the fixture, has a film capacitor parallel attached to it, with correct voltage rating and 0.22 uf rating. The question is will it cause any harm in leaving it there, or i absolutely have to remove it? (as I would need to damage the switch to remove it)

No need to remove it unless it was burnt out. Has no relation to the ballast size. It's just to arrest any spark (causing radio interference) that may form across the switch when it opens, same as in a starter.

So do I understand correctly the capacitor stays effective in series with the fixture when the switch is off?
Then it must go, mainly when there is no PFC capacitor in the fixture. The thing is, the resulting current would cause permanent glow discharge in the starter, wearing it off prematurely.

If you want to somehow "protect the switch from inductive kicks", you need to use two capacitors: One from the line input side of the switch to the Neutral and a second one from that Neutral connection to the load side of the switch. For any fast transients or RF from the contact perspective, these will effectively becoma like in series. But with the switch off, they won't pass any power to the load side.

But I doubt the 170..220mA would be of any problem for the switch at all.
On the contrary, the huge spike currents when the dapacitor gets sharp discharged into the switch just turning ON may actually cause more wear to the contacts than the arc at switch off.

I removed the capacitor. Just before turning it on, I realized there was some grime on the capacitor, and removing it, I found out it was actually 175v peak instead of 275v.