Well I think there are two possible ways for your two last sentences (depends on what do you want to say): They should arrive in about 5 minutes. OR They should arrive about 5 minutes later. And, respectively: They'll arrive in about 5 minutes. They'll arrive about 5 minutes later.

Corrections: They should arrive soon. They'll arrive soon. They should arrive in about 5 minutes. They'll arrive in about 5 minutes.