Is it done? Some OOBs come with a couple of types in the formation German Marder & Marder+ for example but what about other countries Russia with its variety being a prime example.
Take Russia choose your taxi type BTR BMP or MT-L & keep it in the family so would not apply to most OOBs. Would it now be reasonable to have a couple of types in one Mech Co maybe a 3rd for support elements.
Example using BTRs buying as much to win as anything.
2/3rds 70 or 80s with low penetration gun (+ with GL prefered

)
1/3rd 70M or 80As with high penetration gun or even 90- as TI so can lead the way & see whats happening once the dust flies in a fight. What you buy is dictated by opponents taxis as in can you kill it.
Support elements like MGs GLs ATGMs MANPADS could have basic 70, 80 or even 90 though thats a plush ride for them I feel. The rational is its support so stands back therefore the improved fire control keeps it effective & the ATGM covers any surprises.
Same can be done pretty much for BMPs & MT-Ls which makes me think they may mix them a bit.
However I personaly loath AFVs with ATGMs tacked on why not drop him off & resume taxi service? Its nothing to do with cost more the headache of deciding what hes going to be weapon platform or taxi, have found they are quite effective running support around though. A combo of 2 support units & the APC is a good amount of firepower in 1 hex against multiple threats, just dont stay long.
The thing is though is this just a force to win with or does it happen will they field a mix admitedly probably with more ATGM equiped vehicles?