Hi , now this is my first welsh terrier so the way she acts may not be completely typical of the breed!
She is fine with most other dogs , the only time when she has a little grumble is when she is on the lead and large dogs come running up at her. If she is off lead then she is fine , she loves playing with the neighbours collie!
We got her at 11 months and was not house or obedience trained. The house training took a while but she learnt her sit , stays etc quite quickly ( this doesn't mean she always does it though ) . the hardest thing to train her on was the recall , if she saw a rabbit her instincts would kick in and she would just go.
On the protection / guarding theme she doesn't know the meaning of the word! When we'd had her a few months the window cleaners were doing the windows ( 3 big men with ladders wandering around the garden

) and she went out , said hello and went back to bed! We couldn't believe it.
The only time she has been destructive is when we first had her - we used to put her in the kitchen in the evening ( were still working on the house training at that point!

) and she scratched / chewed the plaster off the wall. We think she could here us in the other room. Since then she has been fine - the only things she destroys now are her soft toys which she likes to " de - stuff "

As she's got older she has got easier and lazier ( loves her comforts ) but still has that little spark which i love! She is also a bit dim , bless her! But she makes me laugh every day and i wouldn't change her for the world! In fact we're going to get a wire fox terrier ( hopefully , if the rescue place lets us ) or a welshie again!
Hope this helps a little bit , Kate