I am glad the pics inspire you, i know the loaches use the space as it was meant to be used. Meaning, i'll see multiple loaches each in their own little nook/cave.
Good for you on the rainbows they are aggressive, greedy feeders just like the loaches. I think they make good tank mates. This was my second tank ever setup and i did alot of research on what to put in it. I spent countless hours just looking over fish descriptions at different sites. I knew i liked loaches from the start, then the rainbows started to catch my eye. I already had the rainbow shark in my 20 and he does fine so long as the loaches stay out of his nook:lol: . Then i saw them redline torpedo barbs in a display tank at a fish store in with bows and knew i had to get a few of them. To date they are the most expensive fish i spent money on. I got 2 of them for $33 each and a larger one for $40. I have room for a few more rainbows, i plan on going with the turquoise rainbow (Melanotaenia lacustris). I'm growing a few out in my 20 now, i hope they color up some. I've noticed with rainbows not all grow up to look as nice as the pics show them. I bought my Boesemani's as young adults so i knew what i was getting and paid extra for it. They do make a nice centerpiece fish. It has been alot of fun building this tank.
I do plan on adding some more kuhlis. I have 7/8 of the black variety and 2/3 of the stripped ones. I thought they would school up together but they do not, it seems they are a bit racist. I'd like to even the count and get some more stripped ones, i like them a little more looks wise. But in my tank at least they are the shyer of the two species. I see the black ones swimming out in the open every now and again. Sometimes doing that loachy dance with 3 or 4 swimming up and down the face of the tank. They love the flat stones as well. After i knew i was getting kuhlis i took a coat hanger and dug out the edges under the flat stones so they could fit under the base pieces. That worked out really nice for them as they can't dig in the gravel.
If i would advise anything, it would be to get the stone via a local quarry. I had this stone left over from a flooring project

. Compared to purchasing it at a pet store you'll pay pennys to a pound. Plus the selection would be more varied. Some brickyards will also carry a good selection of stone, but a quarry will undoubtedly be your best bet, i'm a bricklayer so i kinda know about these things. They call the flat kind of stone flagstone, its a general description. The kind i got is newyork bluestone. My quarry has 15/20 different kinds of flagstone to choose from. Its the same place i got my gravel at as well. 2) 5 gallon buckets for like $7/8. Thats by far better then from the local fish/pet store.