Most of the substrates for planted tanks are like a coarse sand or fine gravel but they'll often have some larger pieces mixed in.
Kribs are much hardier than rams.
This is the best way to think about aggression: problems arise when you mix two aggressive fish. Non-nippy tetras, hatchets, rasboras, danios, etc are all pretty passive fish. They're also fast swimmers. If your rams or kribs breed, they'll chase fish like these away, and the fish will get away because they're so fast. It's a good mix. However...say your kribs have fry and the ram wanders nearby. The ram isn't fast but is aggressive. Instead of leaving, he might decide to stay put and fight back. Then, you've got injured or dead fish. Cichlids can be aggressive towards other cichlids even when they're not breeding. I tried mixing a German Blue Ram
with my female Kribensis
in a 29g tank and the krib attacked the ram right away. She would have killed it if I hadn't removed her. The male krib is in that tank now, and he certainly doesn't like the ram but this ram is passive enough that he just avoids the krib most of the time. So, to sum it up, if you've got a potential breeding pair problems are guaranteed if you've got any other dwarf cichlid in there. Even if you don't have a pair, problems are a definite possibility if you've got more than a single dwarf cichlid.
Note that this doesn't apply only to cichlids. I have a Dwarf Gourami
that's somewhat beligerent and will sometimes get in scraps with my kribs, but he generally knows when he's lost and backs off before any damage is done.