I'm not 100% sure on 6 but I do know they will stack up to 4, as that's how many party members you can have out at once. An easy way to check with the Sephira is to fight something with a known base exp (Single Bosses) will be and apply some basic math.
It's also important to remember that even though they say, "Receive 1.5 times as much Gald from battle". Each only only gives a 0.5 increase to the total Gald earned.
1 Sephira = 50% increase = Base x 1.5 2 Sephira = 100% increase = Base x 2.0 3 Sephira = 150% increase = Base x 2.5 4 Sephira = 200% increase = Base x 3.0
Blue Sephira is a lot easier to remember and calculate as well as each one gives double of the base exp.
1 Blue Sephira = 100% increase = Base x 2 2 Blue Sephira = 200% increase = Base x 3 3 Blue Sephira = 300% increase = Base x 4 4 Blue Sephira = 400% increase = Base x 5
Again not sure if more than 4 will stack, but it should be easy to test.