There’s atrium, asylum, arboretum, auditorium
Compendium and modicum and rostrum, crematorium
And coliseum, quantum, condominium, euphonium
And album, acetabulum, museum, pandemonium.

There’s maximum and minimum and optimum and medium
And opium, opprobrium, colloquium and tedium
Colostrum, serum, sputum, sebum, nostrum and meconium
And sternum, talcum, labium, ovum, spermatogonium.

Caladium, nasturtium and laburnum and geranium
And sacrum, c(a)ecum, ischium and tympanum and cranium
Consortium, memorandum, and symposium and podium
Desideratum, datum, vacuum, ultimatum, odium.

There’s pablum, perineum, paramecium, petroleum
And locum and inoculum, lyceum and linoleum
And tritium, deuterium, trapezium and trillium
Mycelium, flagellum, endothelium and cilium.

There’s quorum and decorum, mausoleum, moratorium
And premium, per-annum, honorarium, emporium
And pendulum and forum, fulcrum, speculum, bacterium
And cerebellum, plenum, sum, curriculum, delirium.

Gymnasium and stadium and magnum and terrarium
Solarium, momentum, myocardium, aquarium
And scrotum and factotum and postpartum and continuum
And duodenum, referendum, rectum and residuum.

(ADDENDUM)
To plural them, heads swirling them, “What single rule? - please answer, Pa”.
My dictum, “Don’t inflict ‘em with erratums or chrysanthema !”