No and no... If you want to offer your services you make a collaboration forum topic and let people come to you, or you reply to those collaboration topics that are looking for people and offer your services. Under no circumstances should you contact members directly, either via PM or status update, unless that member has specified in a forum topic that you should do so. If, on the other hand, you are interested in something the user has done or created then you should post in the game or marketplace topic and request that they PM you about any offers etc...
As for hijacking threads, that's just rude...
As with all things forum related, if in doubt
report it! The staff here are all friendly and don't bite, and we'd rather have a few extra reports to deal with than see people getting upset because a minority don't read (or worse, DO read but ignore) the rules.