I don't have experience with Welsummers, so I don't know if pullets naturally have long tails. However, I can tell you that your bird is definitely a pullet. Welsummer males would have different coloring by this age, and would have larger, redder combs and wattles.
On a breed like this, the shape/size of tail is kind of a moot point. Welsummers are easily sexed once they feather in. Males will have a black breast, females reddish. Your bird is a pullet.