Welder
Can I add my own weld templates to welder?
Not yet but this functionality will be added in future realeases.
Can I weld more than 2 objects together?
Not at the same time. You need to weld them pair by pair.
Is Welder altering geometry of welded surfaces
No, Welder only adds additional weld object it does not alter original topology in any way.
Can I change scale and welder rotation after placing one?
Yes, you can modify the scale the same way as any object. After changing the scale make sure to correct the count value in array modifier in order to preserve desired length.