I personally don't recommend any sort of bottom feeder for this tank**. I own this exact one and the substrate that you are supposed to use* (large sharp ceramic pieces) can easily cause you to mis-locate bottom feeders, especially otos.
Fortunately, due to the design of the tank, it cycles quite quickly, as the ceramic pieces provide A++ housing for bacteria, along with the shape with the bubble tube will provide an optimal oxygenated environment for them.
With this in mind I second what laughing said -- concentrate on your girls.
*you CANNOT put sand in this, nor regular gravel. The way the filter works, nothing except large rocks or the ceramic media provided will work properly.
**I also don't personally recommend this tank for bettas (especially males) but females are a better choice.