I usually wire the fuse out of my small scooters
The fuse was semi external on this ebike so when I unsoldered it from the 36v SLA box it that left two wires showing .... that could easily be used to add an extra cell in line.
Tried one 10amphr Headway cell (3,3v extra) then 2 cells (6.6v extra) and it did give the scooter quite a boost on top speed and hill climbing!
The headway cells store easily under the seat.
Different battery technology in parallel help to boost the voltage. But you are putting these cells in series! how long did they last?
Interestingly the 10amphr Headway lasts a similar time to the 12ahr SLA!
when the 36v SLA pack is running down I recharge it and recharge the Lifepo4 as well (which still measures 3.2volts when the SLA pack is starting to feel low)
If your question is referring to the over exagerated myth that you cant have different chemistries in series, its just you have to careful with the capacities. The Headway cell doesnt know the other 36v isnt coming from other Lifepo4s. The electrons dont look any different LOL!