Glad to help! Ok, in respect to filtration, besides the UGF (get back to this in a min), what type of filter(s) do you have? Do you know the gph (gallons per hour) of them? If not, if you know the make & model, we can help you figure this out. Generally, you want 10x the filtration on a goldie tank. So, for a 55g, 550+gph. As your guys are small right now, if you are underfiltered its not a big deal (yet). As they grow, it will become a more pressing issue.
In respect to the pleco, if he is small, he likely is not issue yet but you will need to keep a close eye on everyone. Once they get a taste for a fish's slime coat, it seems to be a situation that only escalates into a tragic one thats easily preventable. So, just be very watchful of your fancies for any unusual marks/sores/strange behavior (particularly when the lights go off). He may never create a problem but you need to be cautious.
Ok, personally, I hate UGFs with a passion. The science behind them is sound and they are perfectly functional for some situations. My experience has been they are far from ideal with messy goldies. Waste & debris gets trapped underneath the plates and its impossible to clean underneath them without ripping your tank apart. As the waste & debris accumulate, water quality takes a nosedive and it becomes difficult to stay on top of it. The functionality of the UGF also declines as waste & debris starts to block the holes in the plates. Whether you want to keep the UGF or not, will be up to you.
Thats wonderful that your son has an affinity for the fish!!! I am glad you discovered something that he can connect with! Keep up the good work and do not hesitate to ask any questions!