Welcome to Fishlore!
It sounds like you've got an excellent setup. One warning regarding the guppies and platies: if you get males and females, they will breed. This could quickly overload your tank. If you don't want them to breed, just get one gender (both are pretty easy to sex, so this shouldn't be a problem).
Other than this, I can see nothing to suggest/change about your chosen stock.
A couple of suggestions:
I would cycle the tank without fish. (You know about the cycle, so I'm not going to go into detail. If you have questions, feel free to ask)
The loaches and cories should go in after the tank has been fully set up and cycled for quite awhile. I would wait at least a month. Both species are susceptible to changes in water quality, and don't do well in unestablished tanks.
As far as sand goes, I'm not sure about how much to use. I usually put enough to get an inch or two of sand in the bottom. Yes, loaches love sand, so that should be good (I use eco-complete, which is similar to potting soil). Loaches also like places to hide, so some
driftwood, rocks, or resin decorations would be a good idea.