Sure, you can put a grommet in the rim to convert the Presta hole to Schrader (there are both metal and rubber options). But I disagree about necessarily getting a flat quickly if you didn't do that. When I got a new rim for our tandem years ago I assumed the hole was sized for Presta since that's what the LBS told me. Used it for over a year without any problems, but then I decided to convert it to Schrader so it would match one of our other bikes. Well, it turned out that no modification was needed - the Schrader valve fit ok in the rim already and I'd been using Presta valve tubes for the last year just thinking the valve hole was a bit oversize. It was a very snug fit with Schrader, but that was clearly the design goal once I examined it closely.

And I've ridden bikes with a mix of Presta and Schrader a number of times - usually because there's a problem with one of the wheels/tires and I grab one off another bike temporarily. And my Bike Friday came with Presta valves on the bike and Schrader valves on the trailer so I need to carry a pump that converts easily.