Well not everyone has big bucks to pay a coder to build such huge script. You should be just fine with Mambo and if it takes off and you start making some money, then do consider hiring a coder for the site.
I run a community site with mambo and I say that it is very capable of handling a site like myspace. phpbb integration into mambo is a little rough, especially with community builder. The logins start to become a mess and you have to apply some hacks for profiles and all as well. I use simpleboard and it is quite good. You can add all the rest of the features to mambo instead of relying on the forum script to supply you with them.
Best of luck, I think you made some great choices!
yea, its basically what you are more comfortable with. Each one has its perks though. I love the mambo community, its wide selection of quality modules and components, and I personally and a fan of the backend administration. The templating could be any easier as well. I tried alot of them out before making the decision. Drupal is a good CMS, just not the one I ended up on. But, that is there reason there are more than one. Not everyone prefers the same things.
Although, I don't recommend anything nuke to anyone.