I have several different needs on my website and I am thinking of using different CMSes - in each case, the one that most closely matches the needs of each page. Is this recommended?
Not recommended. You haven't provided much information, but I can't think of a situation where this would be a good idea. Here are some potential issues:
You will be much better off mastering a single CMS and extending it to meet your needs. Wordpress or Drupal can both do almost anything. Wordpress is easier to start with, but is definitely blog centric. Drupal is more general.
Within each of these platforms there is a lot of flexibility to add different functionality through extensibility and plugins, either your own custom stuff, or freely available options (e.g. in the Wordpress case, forum components like SimplePress and bbPress).
I also have to say that the fact that you are evaluating your requirements against a set of CMS "features" suggests that you alone may not have the technical depth (yet) to take on a project of a complexity that won't be satisfied by any one of them. Consider approaching a Wordpress or Drupal developer and ask them whether your requirements can be met with either of those platforms.
The more CMSes you use the more bug fixes, security holes, and updates you have to keep up with. It also means more training time as each CMS will operate differently. Plus there's always the risk that two or more may become incompatible over time which means a lot of potential work to correct it (and that may require completing removing on of them).
CMSes are designed to handle lots of content types. If not out of the box then through plug-ins. The odds of you needed multiple CMSes is small to begin with and with enough research and effort you can find one that achieves your goals.