Is Porygon2 a Viable Defender Now?

That sounds awesome.... Lucky combination there it sounds like. Go with whatever pokemon you want!

With hidden power (especially yours), Porygon2 is now much more potent since the damage multipliers have been updated; an opponent will go in blindly they first time through not knowing what hidden power porygon2 may have (or charge beam); and then, depending on the typing, may only be able to narrow it down to a few options (in your case, if machamp came in, an attacker could deduce that, due to the hidden power being super eff drive against machamp, it's either flying or psychic type).

If you have stardust that you wish to spend, I'd go for it. Let's say that this porygon2, for whatever reason, becomes less valuable as the new meta begins to solidify. In this case, you will still have a near perfect porygon2 waiting to evolve to PorygonZ in gen 4!