CREST with GFNFF and comparing with GFN2-xTB level. about crest HOT 1 CLOSED

There are multiple problems if ensembles from different levels of theory are compared. One is that the energies are not comparable. The idea to re-rank one of the ensembles at the other level is correct, but keep in mind that -screen does an entire re-optimization of the geometries.
However, in the -compare tool you must provide ensembles including conformers and rotamers (typically the crest_rotamers.xyz files) because you must ensure that the same rotamer for each conformer is compared. Otherwise this could lead to artificially high RMSDs. In your case it would therefore be better to use -mdopt instead of -screen to avoid a sorting of the file, together with the crest_rotamers.xyz file from GFN-FF. -compare does a sorting and identification of the correct rotamer automatically.

To breifly answer your other three questions:

1. You can sligthly increase it (maybe to something like 0.25-0.3 or so), but there is no general rule how it should be for larger systems
2. Working with thresholds always poses the problem of "false positive" results. The sorting is -by construction- only correct within the limitations of the thresholds. At large RMSD thresholds everything will be identified as identical at some point, which of course is wrong. 1.0 A is too much, keep it rather small.
3. This is really system dependent. In some cases the GFN2 conformers are better, in others the GFN-FF conformers are better. For complicated cases you should re-optimize the best conformers at DFT level and then decide.

from crest.