I'm missing the plastic cap/bolt that goes on the spindle (?) on the non-drive side of my my ultegra sl crank. i bought a replacement but I got it for 6600 instead of 6601-g thinking it would be the same, of course it wasnt. would it be a problem to ride without it until the new one comes in? thanks.

That cap is only necessary to set the preload on the crank on initial installation. So if your crankn isn't loose and it's not because of incorrect preload and the crank is already secure then you don't need it.

thanks for the info!