FAQ for common scenarios

Just having a bit of trouble understanding the API... the tutorial and cargo docs are great but I feel I need to do a bit of head-scratching to figure out some things like this:

* how to get all entities that have a certain component?
* how to get all entities that do not have a certain component?
* how to get a specific component from an entity?
* can I have multiple dispatchers (e.g. to run at different frequencies)?
* if I'm not using parallel, should I use dispatchers anyway if I have multiple systems?
* can entities have multiple instances of the same component (like two Transforms) ?

Maybe there can be a FAQ style page in the docs that asks and answers these (and more)...