It is completely possible to feed from one side even exclusively, you might look a little lopsided at times though lol. Babies do generally prefer one side over the other to some extent, even I have a prefered side (because of the arm used to support really) and sometimes it is even just a temperary thing. I would just keep doing what you are doing, my DD1 went through a fairly long phase of only properly feeding from the left and I can already tell DD2 favours that side as well. Supply is driven by demand so if they are demanding more or less on one side, then supply will follow suit. Blocked ducts and mastitis shouldn't be an issue as long as you don't cut one side off cold turkey after feeding from it regularly.