You can not say less then 10 pesukim for each malchiyos zichronos and shofaros. R’ Yochanan ben Nuri says at least three pesukim for each.

You do not use pesukim that are about punishment.

You start with pesukim from the torah and finish with pesukim from Navai. R’ Yosee says you can finish with pesukim from the torah.

Rosh Hashana 4:7

Blowing the shofar is done by tefilas Mussaf. Hallel on Yom Tov is done by tefilas Shacharis.