I don't know, I do enjoy Venom and Carnage.....but because of their popularity they just might explode everywhere...so I really don't know....IMO its more important to have Gobby, Doc Ock, The Lizard, Kraven, and many of Spidey's rouge gallery than the symbiotes. Although it might be cool to have Eddie Brock and Cletus Kasady as secondary characters....but I am really unsure.
Carnage would be hard to do in Pg-13, just because of his violent nature. He's a graphic character.
Venom could work in a movie, but to make him work well, they need to do what the 90's TAS did and SSM did, and that's build Eddie Brock up first. Brock, in the comics, was basically just thrown in with a very lame reason as to why he hated Spider-man. Both the following cartoons improved upon that. Now we have a lot of material you could use to make Brock a compelling character first, and then introduce him.
Personally, I think it would work best if you had Brock as a supporting character for at least three movies, and then introduce the symbiote in 4 and Venom in movie 5. I think it could work if you do symbiote in 3 and Venom in 4, but I'd like to see other characters first, and give the general public more time to forget Spider-man 3.
Again, the trick is, they need to keep Venom to the basics. His first story in the comics (aside from his lame introduction) was actually quite good. I liked the dynamic of Venom stalking and toying with Peter. It makes him menacing, and visually Venom could look amazing on screen.
But don't try and cram Venom and Brock in one movie. Build up Brock. If they're smart they'll have him as a supporting character. They don't have to make him Venom, but if they want to, at least we'll have a set basis for the character this time.