Correct. With the high voltage head, you'll be able to run it on a single 4.2V cell, but not with two primaries.
To get the low voltage head, you'll need to get the Quark X AA² or 123, and then mate it with AA body.
Quark X (which come with XM-L) will produce higher otuput than standard Quarks with XP-G. It's really up to you, to be honest. With Quark X LVH, you'll be able to take full advantage of a 4.2V cell since it will be able to produce higher output.
As for the Pro versus Tactical, Pro does not come with forward clicky. Tactical does. You can, however, swap out the tailcap of the Pro with one from Tactical and you'll gain momentary activation, but it means that it will cycle modes when you try to use the momentary activation too fast.
They still sell 1xAA Quark Pro/Turbo with XP-G, but not Quark X. You'll have to get the Quark X AA² and a separate AA body.
Quarks Pro/Tactical are one of the few small lights that will NOT lose lower outputs even when using higher voltage configurations. They come with really good drivers, which are FULLY regulated.