You guys have been great when I got my water tested I did have a look. A different member of staff to previous did insist on helping, and suggested potentially a barb tank with a couple of Kuhli Loaches? I really don't know what to believe with this person. I'm not lying when I say that phrases such as "your tank is quite small so you won't have to worry about doing water changes very often at all" - this may be accurate, but I've seen so many places say otherwise... I was going to do 25% weekly unless emergency action is required. Also received advice such as leaving the light on for only 4 hours a day.
Lots of knowledgeable members here, and we want you to be successful, not give up with problem after problem and dying fish. Most of us have been through this ourselves.
Water changes are absolutely essential in any aquarium with fish. There are benefits that cannot be achieved by any other means (except nature, or continuous flow through fresh water) including filters. The frequency and volume can depend upon the fish load to some extent, but if one starts by recognizing that no matter how many fish or whatever the tank size, the more water you change the better, you will be on the right track. I do 50-60% of my tanks every week without fail, and I have a fish room with 8 tanks currently. This is essential maintenance for the health of the fish, plain and simple. I would recommend you do one water change a week, and aim for half the tank. This is not a large tank, and chances are you will have it heavily stocked because of that. After all, no one wants to see three or four fish in a tank, and provided you do the water changes, feed sparingly, and have a compatible selection of fish, this is achievable.
"Compatible" fish has many factors, aside from just behaviours (aggression, etc). Water parameters, environment (meaning the decor, the type of substrate, wood/rock, plants) and the activity level of the fish species all factor in. Putting active fish in with sedate fish for example immediately increases their effect on the bio-load because you are creating stress and that means the fish were work harder just to "live."
If the barb advice is OK then I would really like the idea of them. They seemed to have such good personalities and I just watched them for ages - especially the tiger and 5 banded barbs.
The only barb species suited to this small a tank is the cherry barb. Barbs are very active fish, which means room for swimming, and being shoaling fish they must have a group. Six is usually suggested as minimum for shoaling fish, but we must remember that this is an arbitrary number and all shoaling fish will always be healthier the more there are. But cherrys are smallish, not quite as active as other barbs, and could work. They are technically soft water fish, but they may manage though it would not be my preference.
You mention Tiger Barbs...this species should have a 30g minimum tank to themselves, and be 10-12 in number. Some substrate fish could work, but no other upper fish without a larger (than 30g) tank. This species is not only active, it is prone to fin nip and having this number in a 30g tends usually to reduce this. And sedate fish are a "no" with nippy fish, just so you know.
The kuhli loaches were nice and active - would these be suitable? I do really think I would like bottom feeders in the tank. The only thing that came to mind is just how quick they move... wouldn't a 60l potentially be quite restrictive for them?
Here again we have a soft water species, and one that may have more trouble with harder water. They also tend to hide, being semi-nocturnal, so you might not find them all that obvious.
I've covered your "Number 1" above.
Re "Number 2," the
Schistura corsica (now classified in the genus
Nemacheilus as
N. corsica) requires a bit different aquascape. A hillside stream with some good water current, and slightly cooler temperatures than many "tropicals" need. But a good match with the white clouds, which are from similar environments. I would not include
Dario dario though, this fish is a slow-water fish, and best in a small group, usually on its own in a 10g works well.
Some fish are best added to established tanks, others not so fussy. This is different from "cycled," and means the aquarium has been running with fish for a couple months or longer. I would agree that
Dario dario is such a fish, whereas the white clouds are fine in newer tanks.