Small World pellets are about the worst on the market. The main ingredient in pellets should be hay, and the main ingredient in these is wheat middlings. The main hay is alfalfa, which should never be fed to pigs over six months old. I can't find the percent of calcium in the product, but it comes from calcium carbonate, and many pigs have difficulty with that. It's also higher in the ingredient list than the calcium source in good pellets (KMS Hayloft, Oxbow), so presumably is higher in calcium.
It never pays to save money on guinea pig pellets. You'll spend far more on urinary tract problems and stones than you'll save over just buying a good pellet in the first place.
Cut the romaine completely out of the diet, and replace it with red or green leaf lettuce. Romaine is no higher in calcium than the other lettuces, but some pigs have much higher urinary calcium when they eat it than others. No one knows why, but excess urinary calcium is a major factor in stone formation, so it's not worth it when other lettuces are just as available and just as good.
They also should have no more than a small chunk of carrot per pig per day -- carrots are very high in oxalates, and those in themselves can predispose a pig to stones. Mine get one small baby carrot each per day.
What is the forage you give them?