I don't think the system locks out conflicting FP/ADR, though I'm not 100% sure.
I can't imagine it not allowing a 5 - 6 FP and a 6pm ADR within the same park. It would be very easy to use the FP at 5 and have tons of time to get to the ADR.
In looking at my schedule, I do have a 12 - 1 Maelstrom FP and a 1:10 Le Cellier ADR (though the ADR was actually made by my friend with me listed as one of the party)
oh... and I have a Peter Pan at 4:25 - 5:25 followed by Cap May at 5:35! That's arrival day and I probably won't even use it, just made three because i was in the system making a 7DMT FP. But the system let me do it.