Actually they would do better with 6 of one or the other, rather than 3 of both. (The schooling behavior as well as comfort in numbers is expressed with higher numbers of the same species of schooling fish.) I would find some way to make a decision or of course, get a second tank when you can.

(Multiple Tank Syndrome is what we call that urge for more and more tanks.

)
The problem with automatic feeders is that if they break they can dump large amounts of food into the tank, from what I've heard, but I suppose you can try whatever method works best for you and your situation.

Make sure to not overfeed, because most fish have a stomach about the size of ONE of their eyeballs.

With either of those fish choices the tank would be pretty well stocked. No worries about either of those types of fish not getting enough food due to hogging by another, they are all active enough to push their way to the food.
Do you know about the
nitrogen cycle?
http://www.fishlore.com/WaterChemistry.htm http://www.fishlore.com/NitrogenCycle.htm
Make sure to cycle the tank BEFORE you put fish into it, please.
It is very good of you to be asking these questions before you get the fish, so I applaud you for a job well done.
