It is ok.. smaller plants will uptake less nutes, and you won't be running your res with high nute dosages at that point, so your drift should be minimal - a word of advice if you are going to small res.. learn how to properly set your res PH and then don't even check your PH. Don't mess with it.
When you do a top up (maybe at the end of the week) check your res PH.. if it's at like 6.2 and you are adding a gallon.. ph the top up stuff to like 5.5 .. You don't want to do anything but bring it down a little bit. When you are transitioning your nutes from phase to phase, I always clean and change my res.. but I know alot of guys who don't - and it's not hurting them. I used to run 2 and 3 plants out of 10 gallons.. 2 plants was always better than 3. I would get pretty much the same weight but the plants in the res with 2 plants always seemed to do better. Grow bigger, faster, fatter..
Sounds like you are getting some good advice already too.. good luck.