Oxbow or KMS hayloft are the only pellets we recommend. KMS is only available in the US and can only be bought online, the cost of the pellets is low but the cost of shipping will vary depending on your location, but it is important to add the shipping and pellet cost together and then figure out the price per pound, and then do the same for any other pellets you are looking at to be able to compare which is acutally better value for money.
Guinea pigs over 6 months old should be given adult pellets. The main difference between young pellets and adult pellets is the calcium content. Young guinea pigs need extra calcium for their growing bones (but you can also give them extra calcium by feeding them high calcium vegetables/herbs), whereas if extra calcium is given to adults it can increase the chance of them forming bladder stones or urinary sludge.
So given you have one guinea pig over 6 months and one under what you can do is feed both of them adult pellets, and take the younger one out once a day and hand feed them a high calcium vegetable/herb such as a couple sprigs of parsley.