Database Software

Hi, I would like to know if one can have multiple databases running on a single RAC cluster, we have several databases in our shop and would like to consolidate all of them into a single 3-4 node RAC cluster running databases with 10.2 and 11.1 versions.

I am newbie to RAC and would like to get some clarification if anyone has done this, google search comes up with few hits on this topic, so obviously this is not doable.

In our case we have one database supporting critical applications and few other not so critical but are used very extensively between 9-5, so what is the use of RAC if I cannot consolidate all my databases into one cluster, or if I need a separate cluster for each of these critical databases?

I have been all the Oracle docs that keep repeating one database multiple instances and one instance-one machine-one node, they don't even advise running multiple instances on a single node?.

I appreciate any insight.

Thanks.