My pilot licence lists 707-720-727-747-DC8-LearJet + light aircraft.
I could fly them all IF I take a proficiency check (simulator) in ONE of them every 6 months.
And for EACH separate type, a simulator check every 12 months...
This above is for captains, FAA rules.
For first officers, the simulator recency is 12 and 24 months.
xxx
Through my career, I hardly kept current on TWO airline types.
From the 1990s until retirement 2008, I stayed current on 747.
However I moonlighted as ferry pilot, often 727, rarely 707 or DC8...
And maybe occasionally add the LearJet to make a THIRD one.
And my little Piper L-21, makes FOUR...
xxx
My opinion... well, yes you can be multi-type qualified.
To be sharp, I admit, you need that simulator retraining from time to time.
But you can jump on any of your types. Generally not a problem.
Like swimming, you do not necessarily "forget how to"...
xxx
Happy contrails