Wow, that's more than just one question.
Well the reason why they don't have Romero up yet is because he's still playing A ball. Usually a pitcher with his accolades will look pretty good there. In fact, I doubt we'll see him until maybe a September callup in 2007. They don't want to rush him.
Taubenheim has looked decent, but yeah I don't see him as being any more than a reliever in this league in the long run.
The Jays' bullpen, with the exception of Miguel Batista, was fantastic last year. I know it was the best statistically in the AL East, and one of the best in the AL. This was primarily because of Jason Frasor, Justin Speier, Scott Schoenweis, and Vinnie Chulk pitched great. Frasor's been one of our top relief pitchers for almost 2 years now, and he's looked much better recently, I doubt we'll see any more problems for him. As for Chulk, well he hasn't looked great. We'll see with him..
Scott Downs (as long as he isn't overused) and Rosario should also be fine. Pete Walker is also on the DL right now which is also hurting the pen.
I simply don't think they need to bring all of those young pitchers up right now at this point. I think they plan on leaving League in AAA until September as they brought him up way too early last year, and he was blown out of the water. He has the stuff to really be a great setup man or closer in this league though, and it would be best not to rush him, IMO.
As for who's the best out of their young prospect pitchers, well I'd have to say Ricky Romero.