The only way non-cycling-specific footwear has mattered to me so far is when the super-deep grooves of my Nike Free running shoes align themselves with the "cage" of my MKS Touring pedals and prevent my feet from rotating at all. It's funny when plain pedals and running shoes conspire to give me less float than any of my clipless setups.

Well, there's another problem I've had, and that's when I tried using half-clips on the same pedals while wearing shoes with lugged soles. It was awfully difficult to flip the pedal right-side up and stick my toe into the half-clips without lifting the sole off the pedal, which usually ended up pushing the pedal upside-down again. Again, it became more difficult to use than my road or MTB clipless setups, and I took the half-clips off. They're sitting in a box now.

Clipless isn't that bad for my commuting, so I'd say to seriously consider it for your short triathlon.