A prefix never names the subject — it qualifies it, and it attaches with no combining vowel at all. Hold one word ending still, swap the front, and watch the whole clinical picture change. Then spell the joins that assimilate, and prove to yourself that a prefix has nothing to say about how a word forms its plural.
Step 1 · hold this ending still the root and suffix — the subject and the verdict
Step 2 · change only the front the prefix — the qualifier