Ok, you are an events planner and can't see the flaw here?
This is a one year old. IF you are SO into a b-day party; and it will have to host kids of all ages; then, PICK A IDEA. Let's say....host a puppy party. It CAN cover all ages, on so many levels...and I promise, no 2 ppl will dress, act, or react the same.Good for you to think ahead. But, pls, spare yourself...don't over think this....all you really are after is 2 min with your child and the cake/cupcake/cookie/jello...etc.
This should be a no brainer for you....
I also have a son w/ an Oct bday. we do costumes w/in the theame....I give the kids (only the same number as his age) t-shirts that can be or ar already dressed up. I am about to host FOR FUN party and it's theame is penguins. I have visors and tee shirts to help make them feel the part...Ice blocks and squid on a stick...you are over-doing it and wasting your energry and money.
FYI...Halloween is my favorite holiday t play up....A one yr old is too young to understand it...wait 4 years...then the safe family fun ideas of the holiday will be more or use and entertainment.