Neither romaine or cilantro are high in calcium, but everything should be fed in moderation. Spinach, kale, broccoli and parsley are high calcium greens. But too much green veg can cause watery poops and bloat. Cilantro is high in vit C which is great for piggies. Romaine is mostly water with less nutritional content.
We usually alternate our leaves through the week, we feed cilantro 2 days a week as it doesnt last long, then a mixed salad with a bit of spinach 2 days a week, then romaine or little gem lettuce 2 days a week as it lasts longer, along with a daily slice of pepper and cucumber- and on weekends something different like beansprouts and sweet potato.
There is an excellent feeding guide on the forum somewhere.
Based on what veg you are feeding any excess calcium is likely to be coming from either the pellets or the water, though all piggies pee out a bit of calcium, its only if its gritty its likely to be a problem. Filtering the water and feeding a grain free pellet are good options for reducing calcium rather than restricting the sort of veg you mention here.