It's best to not mix species, so if you do get another one make it a whites.

A 30 or 60 day quarantine period before putting your new frog in with your current one is a good idea to help ensure that your new addition is healthy. You're planning to upgrade your tank, so you should have a spare for this purpose.

You can also keep them separated until their sizes are closer enough to house them together if you can't find one close enough to your current frog. But you can also wait until you can find one closer in size, your frog can live on it's own for the rest of it's life without any issues.