“Low pass” means only that the frequency response curve begins at unity gain (at 0 Hz) and drops to -inf gain at Nyquist.
That doesn’t define any other characteristics of the response – how quickly the energy drops off, or resonance, or distortion (or other nonlinearities).
RLPF is a good “baseline” linear filter – clean and reliable. But if that were enough, then VST plug-in companies wouldn’t spend research money emulating the “dirtiness” of analog filters. “Dirty” filters are musically useful.
As for the special-case filters, if you don’t need them, then don’t worry about them. I’ve never used SOS for a musical purpose. You can calculate biquad coefficients matching RLPF, and then SOS can give you the same result as RLPF, but there’s no need to do that.
(But, e.g., HPZ1 is very useful for measuring changes in control signals – for example, if you want a trigger when a Phasor or LFSaw resets to its starting point, HPZ1.ar(phasor) < 0 does the job.)
There may be some subtle differences in frequency response between BLowPass and RLPF. I never investigated that.
Lowpass and low shelf are totally different frequency responses. They aren’t compatible at all.
For controlling the low end, yes, you can use a low shelf with negative gain. (I usually use a highpass filter with some resonance.)
Typically, a low pass filter’s gain at the lowest frequencies is unity – there is no way to force a (R)LPF to boost or cut the lowest frequencies.
I’d expect not much difference unless resonance is high.
hjh