I feel it's an issue of scale.
Take RMX.
Change the RMX professions to SM professions
Change the RMX races to SM Races
Change spells to Psions
Change the skill set, mostly on the choices of secondary skills.
Change the combat tables over to sci-fi weapons.
Change the example creatures to Sci-fi creatures.
You now have an "essential core"
But SF is big on tech and gear, it's not "equivalent to magic" that's psi, it's endemic, it applies to all characters. Your Pilot, your SF Hero and your True Telepath all use tech. Weapons, scanners, shared information gathering (internet), messenging services, etc.
Add on all the portable technology (Equipment)
Add on all the networked technology (internet, communications network, power network)
Add on all the Vehicles (Unlike fantasy, the car/hover skimmer/aircar/gyrocycle can't be written up as a creature and then otherwise ignored, there are no rules I'm aware of for boats or wagons or chariots in RM.)
Could likely get away with no space ships for SMX, condense it down to a price list "Interplanetary ticket 200 credits, intersteller ticket 1000 credits." and do it in a latter suplement. Space ships are key to "Space Opera" not "Sci-fi" genericly.
But while the italics section would morph RMX into SMX in bare outlines, in the same space, the bold items that need adding would beef it up to perhaps twice it's size. . .and I have some doubts that SM could be done with as few skills as RM. (Perhaps with super uber skills, like "Engineering" that applies to everything from building a skyscraper to fixing a hyperdrive to reparing a pocket communicator, and "Science" as in everything from biology to physics via psychology.)
I just don't see how you could pull off SMX in the same size as RMX. . .I could see pulling off SMX in the same page count if all you reporoduced was the different and new materials, cutting all the mechanical stuff that stays the same, but if not cutting the mechanical stuff, what do you cut?
One race, one profession, no psions, no spaceships, no vehicle rules perhaps? That might work out, but people might find it a little too light on actual content. . .It would mean that to get a really meaty SMX they'd need to get SMX and 3 SMX "Extensions" adding in that material to get a really meaty, detailed SF system. (I think the mechanics pages are roughly 3 expansions in size.)
I guess that becomes a marketing call, is it better to release SMX as a mechanics free add on of new SF material for use with the RMX mechanics (Cost of $5 + $5 = 10$ for PDFs) or to get a mega stripped down SMX with mechanics, and then need to buy 3 SMX expansion packs to get it up to the same level of "completeness"? (Cost of $5 + $2 + $2 + $2 = $11).
It would work either way, and I really don't know which one is the better choice. I do see the "Why do I have to buy RMX to use SMX" problem, but it would let you get more bang into that initial book. The customers who end up buying both systems do ask the opposite question of "Why is it I've paid to get reprints of the action round sequence something like 4-5 times now?". . .back when I was buying RM2 and SM2 I often asked myself why I kept paying for that section of the book over and over again. (I think it was in CL, AL, SL and SM). That was the worst, but I think combat and maneuvers got repeated 3 times in there too.