The a in -agenous and, in the vast majority of cases, the -agin in -aginous are part of the word to which the the suffix -ous is added to make an adjective. These are not English suffixes, but Latin or Greek, though they may appear in English words with origins in those ancient languages and could still form new words today.
x + gen + ous
Further dissecting -genous yields -gen, from Greek -γεvής (-genēs), which means produced by or producing whatever the first element is. Think gene, generate, even genitals if you’re so inclined. “Element” is sometimes quite literal: hydrogen (1791), oxygen (1790), nitrogen (1794), halogen (1842) which, at least etymologically, generate water, acid (Gk. oxys), baking soda (Gk. nitron), and salt (Gk. hals). These names were all coined by French chemists shortly before their adoption into English.
You can form adjectives simply by adding -ous: hydrogenous, oxygenous, nitrogenous, halogenous. These words are accented on the antepenultimate, i. e., third from the last syllable.
Gk. κόλλα, ‘glue’ > Latinized, colla > (Fr. collagène ) > collagen > collagenous
x + in + ous
Latin adjectives are commonly formed from the oblique cases of nouns. In third declension nouns in -go, that means adding -in to the root before the morphological ending. The -go itself is a suffix, though it is not always clear what the root is:
Lat. cartilago ‘cartilage’, genitive: cartilaginis > cartilaginos -us, -a, -um > (Fr. cartilagineux) = cartilaginous
With these two words, the root is clear:
Lat. mucus > mucere, ‘be moldy, musty’, Late, Med. Lat. mucilago, mucilagin-, ‘viscous, sticky liquid from plants’> (Old French mucilage) > mucilage > mucilaginous
Lat. ferrum, ‘iron’ > ferrugo, ferrugin-, ‘rust or its color’ > ferruginous
In contrast to more recent scientific words, oleaginous entered English already in the early 15th c.
olea, oleæ (!), ‘olive’ > Med. Lat. oleagin -us, -a, -um > OFr oléagineux > oleaginose > (from 1630s) oleaginous, ‘oily, like oil; unctious’.
Although olea is not a third declension noun but first, the medieval adjective treats it as one. For English, of course, that is of little importance, since the word was simply Englished from Old French.