3s +3s in serial to make 6s. Dumb idea or no?

So i have a handful of 3s lipos from when i was learning to fly, and dont really wanna use them anymore because they just dont have much punch.

So i was considering connecting them in series. Is this a dumb idea? Should i just not use them and just buy a 6s lipo?

Also what would be an ESC amp range for a 6s setup? will 30A suffice? or should i be getting into the 35-40A ESC range?

Also, just curious, what if i put 2 of the same battery, one a 3s, and one a 2s to make a total of 5s, would this cause weird unballanced issues?

Thanks :)