"Be quiet or go out!"
Translation:Byddwch yn dawel neu ewch allan!
tawelwch!/tawela! from tawelu can be used. (Tawelwch is also a noun meaning 'quiet, a silence, etc'.) There is also tewch/tawch!, taw! from tewi.
No, because that is mixing a chi--form with a ti-form.
Tawelwch neu cerwch allan is fine, for example, as both commands are chi-forms.