On starters I wouldn't recommend using an airlock. You want oxygen to get in there to encourage the yeast to multiply. To do this without risking the yeast a whole lot I would use a piece of aluminum foil around the opening or a foam stopper. I prefer the foam stoppers now since an incident where I fruit fly found its way around the aluminum foil. If the foil is all you have on hand then make sure you seal it tightly to the sides of the starter container.
As for the risk to your starter; I wouldn't worry about it. Like you said CO2 was pushing out and the cap on the airlock kept any large object falling in.
Cellar Door Brewery
Primary: Smoked Porter
Secondary: Flanders Red 2010
Kegged: 10.10.10, Robust Porter, American IPA
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-Sir David Attenborough