what is a plural form of a female deer?

No, it's still just "doe".

For example- "There are 6 doe in the field."

Doe a deer , a female deer
lol

As it is a countable noun, it should be "does" the same way male deer would be called "bucks". However the plural of deer is the same as the singular so I can understand you dad's reluctance to use "does". And it "does" does sound a little funny....

You are correct. Does girls, in a group, surely do be called does.

The plural is 'does', the collective is "doe". There is a difference - compare usage of plural "cows" with collective "cattle".

That is correct!