Angels reveal prophetic truth from God

 This comes out i Daniel 10 as well as in other places such as Ezekiel, Zechariah and Revelation.

Angels can be encountered without our realising

Hebrews 13:2 Do not forget to show hospitality to strangers, for by so doing some people have shown hospitality to angels without knowing it.

This is clearly a reference back to Abraham and Lot back in Genesis 18 and 19 and is intended to encourage hospitality. However, it does suggest that it is possible to meet an angel and not know, for them to appear incognito. Perhaps the idea is of the stranger brining the angels with him.

Angels rejoice when sinners repent

In Luke 15:10 it says that

... there is rejoicing in the presence of the angels of God over one sinner who repents.

If that is read carefully, it must mean thhat God rejooices when sinners repent. Undoubtedly the angels rejoice too.

Angels can be elect or non-elect

1 Timothy 5:21 I charge you, in the sight of God and Christ Jesus and the elect angels, to keep these instructions without partiality, and to do nothing out of favouritism.

Paul refers to elect angels, implying that there are also non-elect angels. Calvin says (Institutes 3.23.4) "The angels who have persevered in their upright state are called "elect" by Paul (1 Tim 5:21); if their steadfastness was grounded in the good pleasure of God, then the apostasy of the other angels shows these were forsaken. For that no other cause can be adduced than reprobation, which is hidden in God's secret counsel." This election must have been supralapsarian (logically before the fall) according to Geerhardus Vos. He suggests this election must be in Christ and in Christ as ruler and head (not Mediator). The elect angels did not fall but the non-elect did, according to the will and purpose of God.

Angels good and bad; some lost their positions of authority, some did not

Jude 1:6 And the angels who did not keep their positions of authority but abandoned their proper dwelling - these he has kept in darkness, bound with everlasting chains for judgment on the great Day.

2 Peter 2:4 For if God did not spare angels when they sinned, but sent them to hell, putting them in chains of darkness to be held for judgment; 

Some angels are good and some are bad, the demons or devils. They are described here as those who sinned and so did not keep their positions of authority but abandoned their proper dwelling (heaven?) and were sent to hell. God has kept them in darkness, bound with everlasting chains for judgment on the great Day. They may seem unchained but they are not really. When were they actually hurled down to hell? Probably when Christ died on the cross.

Angels are not to be believed if they say what is false

Galatians 1:8 But even if we or an angel from heaven should preach a gospel other than the one we preached to you, let them be under God's curse!

Paul is no doubt speaking of a hypothetical situation, although there is every reason to suppose a fallen angel preaching a gospel other than the one ... preached to the Galatians by Paul. As glorious and wonderful as angels are they are only to be listened to as far as they speak God's word.