Michael Douglas as Pym? If they didn't want Pym as Ant-Man, they should have just cast for Lang. Making Pym a senior citizen is just kind of lame, and it does basically remove Janet, unless she's either Lang's girlfriend or just Pym's older wife. Then again, if they cared about Ant-Man, he'd have made it into the Avengers before Quicksilver or Scarlet Witch. If they were going to go with an old guy, they could have at least hired someone cool, like Malcolm McDowell. He's less than a year older than Douglas, and he would be a cooler old Pym, especially if they went with more of an exccentric/mad scientist type character to go along with Lang, which is really the only good thing I can see for using an old Pym in the story. He could either be outright mad scientist, or just a weird scientist whose experimenting with the tech, then a younger guy (Lang) steals it, then they end up either working together or fighting. I guess they could do that with Douglas, but it wouldn't be the same. I can't say anything about his acting skill, because I've never seen any of his movies. I looked at his filmography on wikipedia, I have only heard of exactly two movies that he acted in: Basic Instinct and Last Vegas (I've seen One Flew Over a Cuckoo's Nest, but he was only a producer on that). He could be a decent actor, at this point I'm questioning why Pym has to be old more than I'm wondering if his actor is any good.
As for Rudd, I have no real opinion. He's really only been in bad comedies from what I've seen, but I can't think of anyone that stands out as a good choice for Lang anyway, so I guess we'll just have to see how he does. Still, Marvel is not very good at casting its smaller movies, like this or GotG. I'm sure Ant-Man will at least be profitable, probably riding on the fact that the bigger movies are doing so well and that will help it make some extra money, but I'm really unsure of its quality. I want a good Ant-Man movie, and I'm not quite ready to say that it will be bad, but its not looking good.