In XFAB it’s not possible to change any print parameters, and strategies are created so you could achieve best time/quality ratio.
It’s theoretically possible to change layer size using slc file format.
You can create support structures for your model in Netfabb (be sure to secure all minimum points), then export it as a SLC. Nauta will not let you create support for SLC files.
Do not use layers lower than about 70-80% of what manufacturers uses (the are 60um for 289,779 and 90um for 294, all other 100um) because you have to manipulate other parameters to have successful prints and with Fictor for XFAB you can’t.
Also please remember that regarding quality resin is much more important than layer height. You’ll get best details and surface quality in Precisa 779.
If you demand highest possible quality in SLA, please consider buying DWS J series machine.