Alfalfa/lucerne is very high in calcium and could contribute to bladder stones if a too high percentage of the daily food intake is long term alfalfa. Guinea pigs have a naturally very alcaline urine and are more prone to urinary tract problems because of that.
Please do not feed alfalfa hay to guinea pigs that are not pregnant or nursing. Hay should be up to 80% of what they eat, but preferably timothy, meadow or orchard grass.
Pellets: Alfalfa based pellets are OK if given in measure. You need to slowly reduce them to 1 or 1/2 handful per piggy per day once the guinea pigs have got most of their size and weight from 6-9 months old and need less calcium and calories as their bones and bodies don't grow much anymore.
If you can source timothy hay based pellets, that would be better! Alfalfa is a widely grown animal food crop and therefore cheaper than timothy hay.
50g/1 cupful or handful of mixed fresh veg per piggy per day.
Fresh water daily.