You can do them both, but not get a good location to see Illuminations. A 6:30 PS at Chef Mickeys doesn't guarantee you'll be seated at that time. I'd guess it will be 7:30 at the earliest before you'd be done with dinner, even if you hurry.
It's probably going to take you about 30 minutes to travel by monorail from CR to Epcot, depending on how long you have to wait for each monorail.
Then it's at least a 15 minute walk from the monorail to the lagoon in Epcot. That makes it at least 8:15 when you are starting to look for a place to watch Illumnations. Most of the best spots will be taken by then.
I think you can plan to do both, but you won't have a prime location to watch.
By the way, there aren't very many "seats" to watch Illuminations. Almost every place you have to stand to watch the show, because of walls, trees, buildings, etc.
Greg