So if general Iroh (Zuko's uncle) lives in the spirit world and then Korra opens the spirit world portals does that mean Iroh can come visit Zuko again, and vice versa?
Basically, yes. We're not given any particular restrictions on what allows the spirits to visit the material world, nor are we given any restrictions (to my knowledge, anyways) on humans visiting the spirit world. So they probably should be able to meet again, yes.
There is no way for Iroh to visit Lord Zuko. Iroh meditated into the spirit world from the physical world. If Iroh were to go back to the physical world through the portal, he would be like Aang in Winter Solstice: The Spirit World. Zuko would not be able to see or hear him because a spirit was not his original form.