Hi and welcome. Here are my thoughts;
1) Although some plants do best in fine gravel or sand, plenty of plants will be just fine with your gravel. If you set up another tank in the future a smaller substrate might be nice, but I wouldn't go to all the trouble of changing what you have now. One question is; how deep is the gravel layer? If it's too thin, and you need to add more, you could move what you have into deeper regions, and fill in the gaps with a finer gravel to give you more options. You need about 4cm deep at the front to about 8cm at the back or more.
2) The fish will soon get over the disturbance. Plants are best rinsed in case of any residues of disinfectant used to treat them during importation. They shouldn't adversely affect your water quality/chemistry.
What may affect your water is those seashells. Are they real? If so they will dissolve minerals into the water, increasing the hardness, which is not good for most fish, including the one in your pics. Also I think I can see two Red-Finned Sharks. They may get quite territorial as they grow bigger and start to squabble. You may need to re-home one of them.
3) Lawn/carpeting plants that should do well in coarse gravel are Chain Swords, Echinodorus tenellus or E. quadricostatus. If you added a patch of sand, Hairgrass should also be OK. Others will probably add their suggestions to these.
Some other carpeting plants would need CO2 and/or high-intensity lighting to thrive.
4) Depends how fast you want to fill it :) The 20 bunch deal would give you several species and you would find out which like your conditions; those that really thrive will spread and fill your space, or you can speed things along by ordering more of the successful species. Some may not thrive, but you haven't lost much this way.
I always recommend Amazon Swords, Echinodorus paniculatus or E. bleheri, which seem to do well in almost any conditions, and Aponogetons, which grow from a 'bulb'. This allows them to give you a good display from their previous treatment even if you tank isn't ideal for them; and if it is, they will continue to show well for years. Aponogeton crispus, A. longiplumulosus, and A. ulvaceous are all straightforward, the Madgascar Lace Plant, A. madagascariensis needs a bit of TLC. Pop a fertiliser tablet near any bulb when you plant it, and replace it as often as the makers recommend.
5) Remember you can claim a discount for being a member here :)
6) Dying leaves may just be from the shock of moving in, in which case the plants will soon recover.
They will do better if you separate the bunches. If the bottom of the stems is rotting, trim off the all the bad parts and the healthy parts should recover and take root.
Hope this helps; I'm sure you'll get lots more advice, and don't hesitate to ask any more questions, that's what the forum is for.