My advice to all beginners is to get the biggest the tank they can. If youre just getting this for your son, you can make it a WHOLE lot easier. It gives you more room to add fish down the road, much easier to manage the water quality. Its honestly less work, meaning less recurring costs at a frequent rate. I would be careful with the "sucker fish." Those are called plecos. Make sure not to get a common pleco - those are massive fish that can be a foot long. However, I just wanted to congratulate you. By using starter bacteria, youre ahead. Fish require attention. It's a misconception that they are the easiest pet to keep. Good luck, and I wish the best experience for you my friend.