Interesting excavation.. is anything living in it ? And I do think you'll find those puffers are going to be a problem soon enough. Very aggressive and predatory fish, virtually always kept singly, because they eat companions. If they are very young now, maybe you've been fortunate, but you can't count on that lasting.
And it's also very true, cory catfish really need company of their own species. A minimum of 3, and six is a lot better. Having them alone is really very unkind, it's not what they like at all. Many loaches are also shoaling species but not all of them. I'd have to check if the ones you have are the ones who need friends.. if so, the same numbers probably apply to them as the cory cats. Tetras in general also do best in groups of six or more.
It pays to do some research on the fish you see that you like. Just picking one or two because you like the way they look may be rather unkind to fish that evolved to be with many companions of their own species. It can be quite stressful for them and that shortens lives.
Edit... your Yoyo should be ok on his own, given he has tank mates of other species. But the peppered loach, do you know how big he's going to get ? Six inches.. and he should not be alone. Four is the minimum recommended to keep, they shouldn't be singles. Both these species also need a soft,sandy substrate. Especially the peppered loach, as they sift sand through the gils to extract the little critters living in it.
They should be given live food as often as possible, or at least frozen foods. They also prefer to bury themselves entirely much of the time, and I think the goby probably does that too.. I'd have to check again, it's been awhile since I looked up any gobies. But if the substrate is too coarse, fish like the peppered loach either can't bury or will damage themselves trying to get buried, they use a spine on the fin to help them dig in. They also have an adaptation in their guts so they can get oxygen from air. Pretty remarkable what fish have evolved to survive drought conditions.