The only reason you would update to 6.0 is if you intend to play the console online; there is currently nothing else that requires being on firmware 6.0 as not all hacks are supported on that firmware.
You need separate SD cards for ReiNX and Lakka
No, ReiNX leaves traces in the NAND so the only way to remove those is by making a clean NAND backup in Hekate before any sort of hacking then restoring it
so all the homebrew apps i install etc stays on the SD card right?