I don't see any issues with this stock list. Keyholes are very peaceful so I can't see a problem with the cories unless you can only get small juveniles. If that's the case I'd grow them on to adult size before getting the keyholes just to be safe, or consider a larger cory species like the bronze.
That sounds a good plan to have the hard-water fish (platies) in a separate tank to what will be largely soft-water fish. [Depending on your tap water water hardness levels, you may be able to adjust each tank to meet the various species' requirements if need be - we can help with that if you'd like and can tell us what your CaCO3/ppm or German degrees hardness water hardness is.]
Your proposals sound good, including the creation of caves, hiding spots, addition of wood, etc.
Keyhole cichlids are much better tankmates to other fish than many cichlids, so the proposed stocking plan sounds good - more on that in the next paragraph. Cichlids, including keyholes, do like to choose their own mates, though, and this should happen naturally rather than buying two and hoping that they're same sex (sometimes difficult to tell with juveniles) or 'hit it off' as a pair - often it doesn't work out quite so smoothly, so hopefully the LFS will be able to advise on whether it's best to buy more with the proviso that you'll keep two that form a pair or that seem to get on with one another and return the others.
As for tankmates, this is useful https://www.seriouslyfish.com/species/cleithracara-maronii/ . The plan for a shoaling species such as harlequin rasboras sounds good. An alternative slow-moving and non-aggressive shoaling species that might work would be cardinal tetras - but they tend to inhabit the lower third of the tank, so I'd opt for the harlequins in order that the bottom of the tank isn't over-inhabited if you're going to keep cories plus the plec; in fact, I can't think of a better choice than harlequins. Keyholes can get aggressive during breeding, and cories aren't the best for realising/learning not to enter certain territories, so they *might* find themselves getting shooed away repetitively during any breeding spells. If you did opt for cories, I'd err on the side of caution and avoid the nano-sized pygmy/habrosus/hastatus. If you decided to avoid cories, then the cardinal tetras would be a good alternative for the reason I mentioned - they're also good at eating any fallen food. I wouldn't opt for any additional numbers or species than this.