First observation...fish that eat algae tend to be very fussy about which species of algae they will eat. So when you say you want a hillstream loach to deal with "way too much algae," the fish might or might not touch it. The sort of algae I know they do eat is the common green that many hillstream loach aquarists culture on wet rocks in the sun, as one example.
Second, concerning water parameters...GH and pH depend upon species, as some have a preference for harder water than other species, and no mention has been made of a species. Temperature is crucial though, it should be cooler than most "tropical" fish prefer, in the 20-24C/68-75F range.
Now to the flow...if you want to maintain a fish species inn the best health, it must be provided with the environment it expects. Evolution has programmed this into every species of freshwater fish, and their physiology is designed by nature to function best in that specific environment. As soon as you begin to move away from the natural environment conditions, the fish begins to have difficulties maintaining its normal life processes, and it suffers stress, which only makes things that much worse. Flowing water is needed. Anyone can see these fish are designed to grip onto rocks in a strong flow, so provide it. Most (perhaps all) species are found only in shallow, fast-flowing, highly-oxygenated headwaters and tributaries characterized by stretches of riffles and runs broken up by pools or cascades in some cases. This type of environment must be provided if the fish are to be in good health and less stressed.
You cannot keep most Corydoras species in this environment; an exception might be the C. panda, which lives in cooler mountain streams in Peru [the Rio Aquas and Rio Amarillas (tributary of Rio Pachitea), Rio Ucayali system], but this would depend upon the size of the aquarium. Same holds for livebearers really; there is temperature to consider, but if the water flow is too strong for them they will tire and wear out, which means stress and worsening health issues. A long tank can provide space for the "waterfall" conditions at one end with quieter water at the opposite.