That's quite a mix of fish you have in mind there. Here are my thoughts - but these are not recommendations, remember:
The Cherry Barbs, Cardinals, Glass Cats and Whitefins will co-exist pretty well as these are shoaling fish and will stay predominantly in mid-water. A pair of either Gourami you mention will probably fit into your 'mix' quite well also, but could potentially become quite dominant when fully grown (or even less grown) so would need watching. Have you considered the Pearl, or Leeri Gourami? - Slightly less aggressive but a good deal more pleasing to the eye.
I am not familiar with the variety of Angel Fish you mention but you could start off with a small shoal (say, 4) in your tank, with the understanding that as they grow and mature you will probably have to reduce the numbers - especially if a pair forms which is likely to become intolerant of the others!
Golden Rams...a matter of choice here, but they (especially if you can get a matched pair) should fit into the mix pretty well. I (again very personally) cannot see the appeal of the golden morphs when they are compared with the wild-types (the so-called German Blues). A small word of warning - with both types - avoid those from the Far-East like the plague!!! - The European-bred ones are so much stronger and do not come with the built-in obsolescence of their Far-Eastern counterparts.
The 'German-Flag' platies (I'm assuming that's what you mean?) should also be OK - there are now so many derivatives of the basic Platy available - I think these ones are derived from the 'Mickey-Mouse' strain.
You'll be very lucky, I think, to find anyone wishing to part with a Galaxy Pleco so it might be as well to start saving for one.
These do tend to be rather secretive, although gradually become less so with the passage of time but none of the fish in your list would trouble one of them - or they it.
The Denisoni Barbs are nice active fish and would be good additions as youngsters, but can become somewhat boisterous as they grow so would need watching and - just possibly - replacing with more small specimens if this happens.
I am assuming you will be able to maintain a pH level of around neutral (pH7) which would be fine for all the fish - although Platies do come from more alkaline-water sources it is now so many generations of them since they will have come from the wild that I don't think this is so relevant. Likewise with the Rainbowfish, of which there are many different species. Some being 1" 'Dwarfs', right up to relative 'Giants' like the Parkinsoni ones. These Rainbows again can come (but not all) from harder Alkaline waters but I have had little trouble in keeping them in neutral water conditions. The 'Neon-Blues' (Praecox) are nice in a shoal as are the MacCullochi (what we used to know as 'Australian Rainbows' but now I note that this name has now been designated to a different species). The larger ones like the Boesmani etc might end up a little too boisterous (again) for your earlier mix of fish.
As stated at the start, these are only observations and not hard and fast recommendations and (hopefully) others here will add to these notes.
As to the substrate, it's really a matter of preference, sand can be aesthetically more pleasing (I especially like the way Cories scuttle along the surface creating grooves in it) but on the whole I prefer fine gravel as it is easier to maintain (and doesn't clog up the filters!).
Rockwork? - Again, very much a matter of personal taste - so that's rather up to you (I note that you like the Dragon Rock, that's nice).
A word of warning...when your tank is built and you have it up and running - don't be tempted to stock too many fish at once. Fish equals additional load on the filtration system which needs to catch up with every new addition - even if conditions seem 'perfect' beforehand - just remember - More Fish=more food=more 'poo' (polite description)=ammonia increase!
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