Hi,

It's perfectly fine using an electric guitar to work through the fundamentals courses. As you have pointed out already, the principles and techniques taught in the fundamentals courses are indeed applicable to both electric and acoustic guitar. I'm 80% of the way through fundamentals 1 so far on an electric guitar with no issues.

As for building finger strength, Acoustics do take more strength to fret due to having thicker strings but even so there should be no issue learning on electric first and transitioning over to acoustic later if thats what you desire. You still build up calluses learning on electric as well so if you do intend to pick up an acoustic later it will be much easier on you than staring from scratch anyways.

Hope this helps.