Genuine fajitas are made with skirt steak. The most important thing you can do when making fajitas is marinate appropriately. That recipe calls for a dismally short marination time (30 mins to an hour). When I make fajitas I marinate them a minimum of 4 hours, though typically overnight. I usually use a combination of soy sauce, lime juice, garlic, olive oil and salt in a zip-loc bag.
Another thing to be aware of is how you slice the meat. If you bought your fajita meat presliced from a good butcher, then chances are he cut it properly - against the grain. If it's a random supermarket butcher you may or may not have had it cut properly. If you cut the meat yourself, make sure you slice it across the grain, otherwise you'll be chewing some very tough long proteins.
Also, as with any meat, don't overcook it. Medium rare is just fine for a skirt steak.
Some people will substitute a flank steak for a skirt steak when making fajitas. Flank tends to be a little less tough, but not as flavorful. The above marination and cutting guidelines apply to flank steak as well.
Update - I also notice that the recipe suggests 3-4 minutes per side for medium rare. This doesn't sound right to me if the grill is on high heat. Skirt and flank steaks are rather thin and 2 minutes per side on a hot grill should be medium-rare.
Also, make sure you let it rest. I usually rest any steak for 5 minutes, not 3 as the recipe suggests.
Best Answer
I don't think it is close to either cous-cous or rice. It is less starchy than both. Also, it has a unique mouthfeel when you chew on it, because the tiny grains (1 mm or less) pop under your teeth.
I would try getting the real thing in a whole food store or an organic store, or also online. If you can't find it, in these stores, but can get other exotic grains, you can try quinoa or millet.
If you really can't get anything uncommon, I would use durum semolina. Wash it before cooking and cook it with fat to reduce the starch. Also, I would maybe cook it in more water than needed and remove excess water after it has been cooked and cooled, so your semolina grains don't form a creamy pudding.
Maybe tapioca pearls of the extra-small variety can work too, I am not sure because I haven't tried using them and don't know what their final texture is.
You will notice that, when suggesting replacements, I am most worried about texture. The reason is that the flavor of amaranth is very faint and bland. It is best used as a base for other, strong flavors. So you can use anything with the right texture, the flavor in the dish should come from spices and other components.