He's said he's shooting every effect possible as practical and only using cgi when necessary. He's also said he's making different cuts for different ages and will discuss with the studios which ones to use.
And I don't think this is really a part of the series anymore, while in the same universe and while it was originally planned as a prequel, it's now it's own story and has gone in its own direction.
Thats what I heard from some sources.
Certifications for films these days is very different I think anyway. Many films that were a 18 back in the 70's and 80's would be a 15 now. For films to get an 18 certificate now there has to be scenes of torture or hardcore sex. Some films are toned down to reach a bigger audience but I think it will have little impact on this film. As long as the story is as good as Alien was and has plenty of wow moments I will be happy. I think the biggest problem this film will have is too many people are expecting this film to be a direct prequel to the Alien films which it is not.