0db (in red) okay in FCPX?

Sorry no one has gotten back to you yet. Tom Wolsky has this to say in his boo:

The standard audio level for digital video is -12dB.Sound cannot be recorded higher that 0dB (there's no function for it in digital, super loud sounds drop out or distort). -12dB for normal speech, lower for soft sound, higher for shouting. Only very loud transients like gunshot should get near 0, closer to -2dB.

That's about all there is to it. Well, that's all I know and I owe what little I know to Wolsey.

You can use the audio enhancement function of the Audio portion of the Inspector for the clip and let FCPX have a run at tweaking your track but the results are usually ruinous.

Musicians are a weird lot and their audio tracks are often piped through separateplayback systems different from the video screen setups.

I am not an audio engineer! I have, however, taken college courses in electronic music composition. My first project (1989) was a tape music composition. I burned it like the Ponderosa the signals were so hot. It was so hot, (How Hot Was It!) the audio copied itself on adjacent pieces of tape about 4 deep. Radioactive hot. Needless to say I failed that assignment. I recovered and got better.

If you see red — the audio IS clipped. Red means clipped. [If you're an audio engineer? It's you've just been fired clipped.] You're kind of "okay" if you stay within the yellow range (0 to +6db). NOT recommended. FCPX will go +12dB in the storyline. That's a relative gain adjustment. The waveform will display red on any part of the waveform above +6dB. [Increase the size of the audio clips in the storyline so you can see the distinctions.]

Recommendation is:

Adjust audio levels so that they do not pass the 0 line. [This is easily done in the storyline - adjust the gain line downward or upwards until the peaks rest on the 0 line (exceptions being loud sound effects like blasts, gunfire, etc.)]

You have a rather full complement of Logic audio tools in the Effects browser. Scroll down about halfway to Audio, and then for each type of audio (e.g.: EQ) you can find a section of tools under the Logic grouping. Use Adaptive Limiter to increase the apparent Loudness and Channel EQ to shape the sound somewhat. There's also Multipressor if you need a sterling compressor. Many many other Logic tools are in there (you can even apply the Multimeter to monitor your audio and deal with things like Correlation, etc.)

(once you add a logic effect to a clip, in the Audio Inspector you'll find an entry. Click on the small icon at the right edge of the effect entry to call up the UI controls — exactly like being in Logic! er... Logic 9, not X.)

Rely on the user to make the final audio volume adjustment, but overall, for clarity and overall better sound, keep the audio levels "comfortable" to listen to with your Mac's System volume set to about 50% (or a little less since Yosemite) with headphones. It'll be great at louder settings, but let the user decide what that should be.