Wingsley: I'd tone things down to the approximate length/width/height of a Sovereign & the same payouts in terms of phasers, torpedoes / tubes as a standard Sovereign-Class. I'd make it a one-time Wolf 359 "frankenstein fleet" vessel with a two-part MVAM (rather than three,) as it ~IS~ just a Promie "Alpha" saucer-section piggybacking on a Sovereign-Class stardrive.
A partially-completed Prometheus-Class was mated to an incapacitated Sovereign-Class, laid-low by The Dominion, as...(add plot here) and various crew were quickly gathered from other assignments in Utopia Planetia's repair queue on-the-fly to deal with the (add bad guy here.)
Besides, you wouldn't want this ship provoking non-Federation political entities that pick up the Poseidon on long range sensors & wonder if the UFP is initiating an arms race/build-up during a time when they're espousing a return to Starfleet's original Pike-era type peaceful exploration.
You'd barely take this one out of spacedock/drydock except to address the Kam'Jathae / Hur'q, or renegade Klingons/Romulans, or a rare glimpse at a Borg scout.