Unlike a lot of people I make no attempt to socialize with my chickens. I tried it but all them clucks might as well have been Greek and I don't think they understood a word I was saying either. If roosters were going to be aggressive then my would be. Not that there aren't aggressive Welsummers but I've never owned one. In the last 3 years I've have 11 roosters and none have ever shown the slightest bit of aggression. This evening I was in the run, squatting down, to get a better angle for photos, and on numerous occasions was within inches of the rooster and his hens. He never once made any indication that he was upset with my presence.
A good guage in what to look for in a rooster is how gentle is he on the hens. None of my hens exhibit the slightes evidence of "rooster wear". When buying chicks from someone, ask to see their rooster, look at the hens, and if either sex doesn't exhibit what you feel a good Welsummer should look like don't buy the chicks. If the rooster seems aggressive and the hens look haggard, don't buy the chicks.