It'll look sloppy if you just combine. I would suggest adding edge loops around the tube ends you want to connect, combine only those parts and then delete end faces and merge verts so the combined object looks like 1 solid piece instead of one pipe cutting into another one. As for the welds, you could do that in Zbrush, or just have a cylinder around the area you want to "weld" and then smooth that object for the appearance of a weld.