I agree with you that aiming for more RBs with those league position settings sounds like a good idea. I don't have any problem with drafting 4 RBs in the first 6 rounds.
However, if this is a PPR league i would be concerned about this decision. I think that bumps up WR values significantly where you are harming your team if you aren't addressing that position until round 7.
My main tip would be to look at position value and tiers. For example in round 6 in a 12 team league if you are getting an Addai, Moreno, or another lower payoff feature back you are getting good value. However, that spot would be a bit of a reach to draft say a Fred Jackson, TJones, or LT if others have been drafting RB heavy too and I could get a Boldin or one of the SSmiths.
So the strategy seems sound, but always be able to grab a steal at any position instead of sticking to dogmatic drafting.