I have two small (less than 5 inch) leopards, and I feed them a varied diet.
When the weather is warm and I have the time, I put them out on the lawn (no chemicals) under a screen-topped frame and let them eat what they find. Mostly they like clover and weeds, and maybe a little grass.
When they are indoors and there are no weeds to feed them, I use a variety of grocery greens and some tortoise pellets mixed with chopped timothy hay. The greens include collards, endive, mustard, kale, spring mix, etc. Maybe a little carrot or squash. I've use Mazuri, Pretty Pets, and RepCal tortoise pellets, moistened and mixed with finely chopped timothy hay. I try to make the pellets a fairly small part of the diet, say once a week or so. And if you can find it, they really like cactus pads. With the spines removed, of course.
Also I supplement with RepCal Calcium with D3, especially when they do not get much time outdoors.
So my recommendation is to give them a varied diet. Don't try to find one magic food. And don't give them mostly Romaine Lettuce. It's OK as part of a varied diet, but it is not complete by itself. And it is too low in fiber. Your torts will produce runny stools on that diet.
Later, as they get larger, they may consume a higher percentage of grass in their diet.