Ok we are now dealing in a topic close to my heart. If you have varing sizes on the team I say costume to the average. If you always costume to the biggest the girls the other girls will not want to wear that costume. On the other side putting a 3x in a costume to make them feel uncomfortable is not fair either.
My pet peeve in this is you do not have to be an adult small to wear a cute costume often an AXL has a great shape it's just a bigger shape than the adult small. Women are suposed to have curves and it is ok, they do not have to be shaped like boys. Many times you will see a team with 1 girl who is a 3x and the whole team will be swimming in fabric and the girls who are an adult small will look like they have on thier mom's costume. More fabric does not always mean "looks good on a big girl" sometimes more fabrics means makes a big girl look bigger.
One trick is to alter costumes when the come in. Most companies sell costumes in sizes like adult small, medium and so on. This does not mean it will fit like a glove alterations may be necessary. Take in the waist on smaller girls leave a little extra on the bigger girls.
Proper undergarments are a must, Body Wrappers makes great "stretch underwraps" they hide alot and hold the girls tight under the costume. I require all my girls to wear them no panty or bra lines along with no jiggles.
Last but not least girls like sparkly things, no matter how adavanced the team is they still seem to get excited about sequins and rhinestones. Try to see some of the Scholastic World guards from WGI on the field and you may be surprised what they wear. The look is way different than what you'll see at WGI.
Find someone at one of the costume companies that you are comfortable talking to, they will help you out. Ask to speak to someone who teaches guard when you call all the companies have at least one person on staff.
It goes without saying hair/makeup shoes and equipment complete the look.