Within one piece of land, can you turn mob spawning on and off for certain parts of the area?

Yes, you simply need to get a sub-region created around the area you want to turn it on/off with a pe, within the parent region and then put in a pe saying something like "/pe new please turn mob spawning on/off in <region name>" The staff member would then simply put a flag on that region allowing or denying mob spawning!

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And you can do this with land thats not governed by the town plugin?

Any region can have flags applied to it by a staff member.

Whether the specific flags you want are allowed or appropriate is decided on a case-by-case basis, but generally, yes, any region is capable of having its flags modified as far as I know.