Honestly I can imagine if MCSM characters were Steven Universe Characters

Lukas - Steven
F! Jesse - Connie
Petra - Ruby
M! Jesse - Sapphire
Olivia - Pearl
Axel - Amethyst
Aiden - Peridot

Stevonnie
The first time Lukas a Jess would fuse would be one the beach after Lukas learns Jess can't dance.

I could imagine their fusion like this:
- Long, dark hair with blonde highlights
-One green eye and one blue eye
-A red and white striped shirt
-Jess's overalls
-Lukas' jacket

(And yes, I did base this off of PrettyxThexArtist)

Garnet
I can totally see Petra saving Jesse from getting poofed as he stands there and prepares to take it.

I could Imagine their fusion as:
-Short red hair (most of it is in Petra's bandana)
-Jesse's shirt
-Petra's vest
-Some kind of mixture between Jesse's pants and Petra's shorts
-and then suspenders

Opal
This one seems the most accurate. The two have a hard time getting along. Lukas literally almost getting killed, helps them fuse.

I honestly can't really see what theirs would look like.

Peridot
Ah yes, antagonist turns to protagonist. I can see Aiden trying to kill Lukas, getting poofed but (whatever the frick you'd call Jesse and Petra's fusion), then turning to the good side