My 2 girls took about 2 years to hit 4 inches. I am pretty sure they are fully grown now, as they have not grown at all over the last 4 months. Thank goodness, I thought they were going to grow outta their tank. Your frog should be fine if you add one later, I added 1 to my setup that was about 5 months old and my girls were just over a year old. There were no problems and no bullying. They took him right in as one of their own. You should be fine with the plan you have set.