A Bestiary question I'm embarrassed to ask...

[ruemere]

That brings up another question I was wondering about concerning large creatures using reach weapons and having a natural attack. If they were to have whirlwind would they be able to hit everything withing 20' or would they have to choose which weapon to use? As posted earlier a reach weapon doubles your reach and you wouldn't be able to hit anything inside of your reach. Then with your natural attack you would be able to hit everything within 10'. As the feat is written it seems like they would be able to hit everything within 20', unless I am missing something.

So, to be precise, you want to use Whirlwind Attack feat? If so, you just need to follow feat benefit to the letter:

When you use the full-attack action, you can give up your regular attacks and instead make one melee attack at your highest base attack bonus against each opponent within reach. You must make a separate attack roll against each opponent.

When you use the Whirlwind Attack feat, you also forfeit any bonus or extra attacks granted by other feats, spells, or abilities.

1. Choose any melee attack your character is capable of performing.
2. Make that attack against every opponent within the range of this particular attack.
3. You cannot perform any other attacks this turn.

Note that:
If you pick reach weapon attack, you enjoy both extended range and suffer from being unable to hit adjacent opponents.
No matter what kind of additional attacks you may have, you cannot use them - if you were a beholder (free action attacks with eyestalk rays) and made a Whirldwind attack with a toothbrush against a swarm of mice, you would not be able to use your eyestalk rays on the same turn.

Regards,
Ruemere