ummm yes the OP did state that ;
"The only scripture I am aware of is Matt 18:10 -
"Take heed that ye despise not one of these little ones; for I say unto you, That in heaven their angels do always behold the face of my Father which is in heaven."
It sounds there as if we each have a guardian angel from birth...
I can certainly take note of the "shall be" and this is where translation can take on a life / meaning of its own. Depending on how the translator words things we can read into the text things that may not have been meant. Let's look at various translations to gain an understanding of this thing;
International Standard Version
All of them are spirits on a divine mission, sent to serve those
who are about to inherit salvation, aren't they?
Young's Literal Translation
are they not all spirits of service -- for ministration being sent forth
because of those about to inherit salvation?
Berean Literal Bible
Are they not all ministering spirits, being sent forth for service for the sake of those
being about to inherit salvation?
Weymouth New Testament
Are not all angels spirits that serve Him--whom He sends out to render service for the benefit of those
who, before long, will inherit salvation
See these translations imply the angels are sent at a time when the believer nears his inheritance point and does not imply from birth.
Jubilee Bible 2000
Are they not all ministering spirits, sent forth in service for the love of those
who are the heirs of saving health?
and here they imply service for those already saved.
Here are some commentaries on this subject;
Ellicott's Commentary for English Readers
To the English reader it may seem that those who in
Hebrews 1:7 are God’s ministers are here represented as servants of man. It is not really so, for the words properly mean, . . . sent forth (that is, continually sent forth) to do service (to God), for the sake of them who are to inherit salvation.
Gill's Exposition of the Entire Bible
and so the Ethiopic version renders it, "who are heirs of salvation";
nor should it be rendered, "who shall be heirs", but rather, "who shall inherit salvation"; for this character respects not their heirship,
but their actual inheriting of salvation: and the ministry of angels to, and for them, lies in things temporal and spiritual, or what concern both their bodies and their souls
Your concept that God knows who his chosen are before they are even born is not born out by scripture. God made us in his image and that image includes the power of choice. To assert that one is born with their choice already made removes from them their godly image and renders them as automaton. What sense does it make for God to say;
Ezekiel 18:21-23 “But if a wicked person turns away from all his sins that he has committed and keeps all my statutes and does what is just and right, he shall surely live; he shall not die. None of the transgressions that he has committed shall be remembered against him; for the righteousness that he has done he shall live. Have I any pleasure in the death of the wicked, declares the Lord God, and not rather that he should turn from his way and live.
If there was no possibility for us to have the power of choice?