Sorry for the slow response, it's been a week of nothing but crazy work.
I'm almost 100 per cent certain that you can't create a farm team in Yahoo. You could create another "league" with just your farm team players, but then you would need to figure out how to deal with those farm team players in your "real" league.
I don't know any service provider that allows farm teams, but I am certain that there is at least one that can do this. I primarily use Yahoo and have one league in Fantrax.
A dynasty league is simply a keeper league where you can keep all or nearly all of your players. I run a 24 team dynasty league and I love how much I am learning about prospects and the power of patience!
I'm not always thrilled with lmited keeper leagues because it's almost like you are being punished for good/great drafting and trading. It is meant to keep some teams from getting too much of an advantage. I understand the need for this because if one or two teams end up so far ahead of everyone else, then the rest of the league will likely lose interest.
The key really is having a group of owners who are reasonably close in skill. Trading when one or two owners take advantage of a less skilled owner can sometimes throw the pool out of whack and sour the inexperienced owner.
12 Team "Experts" Roto representing Dobberhockey 2nd
G,A,PPP,+/-,SOG,Hits,Blocks,Wins,GAA,SV% - Keep 4
12 Team Salary Cap Roto Dynasty 3rd (G,A,Pts,FPts,DPts,Pim,SOG,H,BS,W+OTL+SO,GAA,SV%)
12F-Ovechkin,Kopitar,P&E Kane,Kessel,Ryan,M&N Foligno,Ennis,Hodgson,Killorn,Horton,Wolski,Morrow ,Bonino