My understanding of "Psychology of Selves" is that it is an analytical approach to categorize and differentiate various personality components. We each have a variety of character attributes, derived from our primary needs and drives. We have developed diverse tendencies, qualities, skills, habits, weaknesses, etc. which can all be broken down, itemized, and personalized. Such an effort could be beneficial, in terms of getting to understand and know ourselves, and as a guide for applying healing techniques.
For example, we might have a hyper-vigilant protector self; a dysfunctional sexual self; a parental self with dependencies upon the children; an artist self who feels imprisoned or frustrated; and an inner child with unresolved trauma. We can work with each of these somewhat independently to heal them. (But of course they are all related to each other).
As we approach inner work from the EC slant, we immediately start to bump into troublesome suppressed feelings. Each of these feelings could be said to correspond to a certain sub-personality. Excessive suppressed fear directly correlates to the hyper-vigilant protector self, for example. In fact, I would say that the fear has created it. Therefore, as we discover a variety of suppressed feelings, we are also discovering what could be identified as sub-personalities, if you were so inclined. As you process and release each feeling, you are healing the sub-personality it corresponds to, in a most effective way.
However, in EC practice, we usually do not emphasize needing to name and personify particular suppressed feeling attributes. To understand that we have excessive fear, and that it tends to make us act out in certain undesirable ways, or draws us into terrifying situations, is usually enough. However, if you like to use sub-personalities, it would be completely compatible with EC.
The way we organize feelings is with the Chakra system. EC is an East/West system. We think in terms of Chakras. Thus, we may have excessive suppressed fear in the Survival Center, to continue our example. Or the Sexual Chakra is blocked. Or the navel nurturing Center is starved and is unconsciously trying to drain energy from the kids. Identifying the Chakra then makes possible other forms of intervention within the EC Process. (Astrology is yet another exceptional method of understanding your energetic make-up that I recommend, that could also work well with sub-personalities).
If you have read the EC book, you did not see the section about beliefs, to which I would refer you. Briefly, we consider that trapped feelings create beliefs, not the other way around. We do not attempt to recondition or replace beliefs, which would be a futile, left-brain, ego-directed mis-effort. If you could alter a belief, the suppressed energy that originally created it would still be there, and still causing havoc. As you release the suppressed feeling, the limiting belief that it created dies away naturally with no effort.
And am I sensing some beating yourself up for no good reason? Intellectual curiosity is a valuable asset, and these are good questions.