Does all of a poem have to be in the same meter?

There is no actual law enforcement agency that forbids poets from creating such mongrel poems.

You can write however you want. Can't say if any poetry professors or publishers will the hybrid poem or that it would be a readable or good poem, but the writer just has to set pen to paper or digital keystroke to Microsoft Word program, etc... to write whatever floats her or his poetic boat.

so you're saying one line could be iambic pentameter then it could switch to a trochaic or dactyl meter of some sort? much like jam bands switch keys during improving. But typically a poem should be in one meter, right?

It seems one meter creates a mor unified impression, hence it is more able to produce a poetic mood.

I have done hybrids deliberately...the meter can be a stick but it is not the poem legs...

There are no such rules for writing general poetry, only for specific types like sonnets, haiku and limericks.

Go for it . . . as others have pointed out, there are few to no rules, and the forms are customary. If you can make it work, then you'll be a better writer for having made the effort.