How much luck have you guys had with drafting rookies at WR and RB and trying to convert them to dual-purpose players? I imagine you would go through one season at one position, then the second position at the other? Or, if you do so, does it retard their development so that they are ineffective at both positions?

Short answer: A lot.

I usually play them at WR if they are going to play a dual role so that they'll hit their max speed.

If I draft a HB for a dual role, I leave them at HB the first season or two in hopes their break tackles develops faster, after that I'll move them to WR for speed.

Speed + Ball Carry > Break Tackle

Remember, Break Tackle is dependent on a player's SP and not a player's ST. So faster players "break" more tackles because they are shifty. All ST allows the RB to do is move the pile.

Ive had a lot of successfull wr/rb guys, but the most obvious one, the one with almost 100 rating in both (christopher gomez in mfn24) is the one who can only produce at wr and not at rb. I even tried a year at both position and it nerfed him down in both positions... Going from a star to a backup stats player.