I disagree, although they can be rather boisterous they should do fine in that size tank but then again you may be right and they could pose an issue when older. All those fish can adapt well to hard water so I wouldn't worry about that at all, most kribs will breed in those parameters no problems.
I think the hardness of the water isn't the real problem. The number and the tank size is the main issue.
OP, I'd urge you to return the yoyo loaches, please, or seriously consider whether you can upgrade them to a larger tank that can house 5 or more of them as adults. I'm speaking from personal experience, not fish care sheets.
I semi-inherited a 55 gallon that is a disaster. For around eight years now, there have been a pair of botia in there that are almost identical to yoyo loaches, perhaps pakistani loaches, we still haven't been able to identify the sub species.
Loaches get pretty big when they're adult, but crucially, they seem to be a pretty intelligent and highly social fish. They thrive in groups of five or more, form bonds and a pecking order, show every appearance of enjoying 'playing'. I love botia for this, they are beautiful, and very engaging to watch. When they're in good conditions, with lots of places to hide and a pack of buddies, they can be quite bold and funny and you'll see them a lot.
But these two botia in the 55 gallon for eight years.. they are not like that. They spend nearly all of their time hiding behind driftwood, you rarely see them. They do something called 'greying out' often, which usually means a stressed and unhappy loach. I'm absolutely positive that they do not show the full range of behaviours that loaches show, because there are only two of them. Loaches form that pecking order, and the dominant one can bully the other, and that bullying isn't dispersed among a few fish, but targeted at the only other botia, who is trapped. Same problem if there are three of them, two of them can gang up on the third. Larger groups mean they can form the kind of group bonds they need.
And now that I've inherited this problem, I'm stuck. Once I read up on them, I really wanted to get them some friends, so I needed to know the sub species. But, they hide so much, and move so fast when they come out, that it's incredibly difficult to get a clear identifying photo of their markings. The markings on different species can also change as they mature, and species photos don't show eight year old fish.. They look just like yoyos, and yet don't have that identifying Y shape. Yet every other species photo I check doesn't quite match either. I thought about buying some zebra or yoyo loaches to go in with them, and see if they would school together, but it doesn't work that way either, the other species could harass the old pair and make it worse. Then make the tank overcrowded to boot. My father refuses to give them away, and how would I find someone who has a group they could join, when we don't even know the species? So they're likely doomed to stay, and remain lonely until they pass. That's incredibly sad, and I really wouldn't want you to see this happen to your yoyos.
Have they survived in hard water for eight years or more? Yes. But have they thrived and been allowed to express the full scope of their natural behaviours? No. Have they suffered because of that? That depends on how you quantify suffering, but I would say that they've endured a certain kind of dull torture. No one wants to be a fishkeeper who doesn't care about the lives of their fish.
and I don't think your tank is large enough to house five or more adult yoyos.