In my experience Keflex shouldnt interfer with your xtc experience. Not sure in terms of if X will decrease the effectiveness of the antibiotics.
If you want to miss a few doses I would take x 2 500mg tabs in the evening/day before (so 1g in one dose) and just miss the next 2 doses on the day you want to roll, followed by taking another 1g the night after or something like that.
Its no big deal if you take 1g at a time with these first generation antibiotics.
Beware though depending on what your taking the antibiotics for, having a bender will probably extend the life of your sickness
x