Would I be able to bring my own tables and chairs and set up near the baseball fields?

Go for it

As long as you clean up your meds and don't damage the grass.you are all good

Yes, you should be able to. You can always double check with portland parks and recreation for the rules. Certain things require permits, depending on the event.

Of course, they welcome small groups.

I would say it depends on how elaborate the part is. You talking two tables and a few chairs... all good. If this is a 10 table party with 80 people you may raise some eye brows and be asked to leave. Small stuff you should be good.