Only channels 1&2 have Hi-Z inputs -, you’d need a DI box to plug a guitar directly into 3&4, Behringer’s excellent little DI20 will do the trick and it only costs about £25. As for the “other” vocals please give us a chance, what mics do you use etc? We’re not mind-readers. Channels 1/2&3/4 have separate switch-able phantom voltage (which is superb well done Steinberg) which could be used to power the DI20 on 3 whilst providing power for a condenser mic on 4.