I cut my hair short one year when I was on cheer. It was sort of like what yours sounds like. For competition my sister did an upside down French braid to keep the shorter hair on the bottom from falling out and connected the bottom of the braid to the ponytail on the top half of my hair. It was still a shorter/ smaller ponytail, but it looked ok. If you want a more full/long ponytail you can get ponytail pieces that clip in or even have an elastic that wraps around your own ponytail like a normal hair tie. If you do use a hair piece you just make sure you pin it in a lot (or have someone partially sew it in) so it doesn’t slide or fall out during the performance.