Can Guinea pigs eat romaine lettuce?
Romaine is fine to feed to most guinea pigs. In other pigs it can cause chalky/gritty urine and can cause problems in pigs that are prone to bladder stones. If your pigs are not prone to stones and do not have chalky pee from it you can go ahead and feed it daily if you want.
I suggest you check the following chart (and maybe even print it out). It shows all the different types of greens and veggies that can be fed to a guinea pig AND it explains how much and how often for each type. Some can be fed daily, some less often. https://www.guineapigcages.com/forum/threads/22156-READ-ME-Cavy-Nutrition-Charts-amp-Poisonous-Plants-List
It is a cos lettuce and all and any lettuce is not the best food for Guinea pigs certainly not in any quality as all and any are mainly water...... far better are carrots, apples etc