S.M.
I believe cold mist humidifiers are supposed to be the safest, but be careful to get tht ekind without a filter or change it regularly or it could do more harm than good. I see no reason not to use a humidifier for a six month old - it has always helped both for the moisture and for the white noise.
Definitesly keep breast feeding as that shouldn\ shorten the length of the infection. But I would avoid medications, even Claritin - check with the pediatrician, but almost all cold and allergy drugs get passed in breast milk and she is too young for these. Keep her upright after feeding if possible, becaus ethe milk may increase the mucous, but do not prop her in the crib.
Colds can seem to linger in babies because they aren't able to clear their throats and noses easily (use the bulb siringe from the hospital). However, coughts can be very dangerous in infants, so pay close attention to that aspect of it, especially if it persists or becomes croupy.