Yes. When the music’s in full swing, peaks should be at or above the zero mark on the X1800 dBVu meters, depending on how dynamically compressed or dynamically spread-out the music is. Every track should not necessarily peak at the same level. Sparse stuff will often need more gain/trim to sound as loud as music that’s very dense… this has to do with the perception of loudness. The top LEDs on the meters is the hard limit. The LED right below that, though, is usually about -10dBFS. So second-to-top LED you shouldn’t intentionally go into, but it won’t technically clip yet until the very top LED.
You also don’t have to worry about noise, exactly, as if you’re doing all this you’re maximizing the signal to noise ratio on the mixer’s master output while not doing anything stupid. The input stage noise is moot, since with SPDIF there is no analog input stage, and the only brand of digital DJ mixers you have any control over the signal level into the analog-to-digital section is on Pioneers. On every other brand of fully digital DJ mixers the gain/trims are entirely digital domain in the DSP and the actual analog input trims are fixed with a big, healthy headroom built in for most signals.
By the way, these SPDIF connections are electrical over (optimally coaxial, but irrelevant at these short lengths) RCA cables, not optical ones that are light over fiber. We all know what you mean, though.
Also make sure you update to all the latest firmwares and check back frequently for those.