Question

Will it damage anything if I connect my guitar directly to the audio interface or do I need an adapter?

Also which adapter is good for connecting speakers to the audio interface on (on "Main Out" of course)

Answer 1

Channel 1 has an instrument in which is made for this purpose. Just plugin your ordinary guitar cable and level the signal with the 1-knob. To get the best results drive the preamp to about -18 db in Studio One. This should be the sweet spot.

This really helps to get good results: https://www.recordingrevolution.com/stop-recording-so-hot-into-your-daw/ and https://www.recordingrevolution.com/are-you-recording-too-hot/. Keep it at about 2/3 to maximum and you're good to go.

Have fun. :)