You look at Guerrero's stats on baseball-reference and he pretty much has more of everything as an Expo. His entire postseason body of work and one MVP season came as an Angel but other than that, yes, his plaque should have an Expo cap. A similar action would be putting a Phillies cap on Halladay's plaque.
At age 43, he's the youngest HOFer ever. Heck, there are active MLB players who are older.