Ok so first the quotes:

Bossk:

and mangler:

So as far as the wording goes when this combo attacks it goes: when attacking you may change one hit to crit from mangler, then bossk wording says that after you performed attack (modified already by mangler) before the dmg is dealed you may change one crit to 2 hits.

Is this a legal combo to change 1 hit from mangler to 2 hits?

Yes. As you said, Mangler triggers first.

Just be aware that the attack has to "Hit" before Bossk's ability can trigger, so his extra damage doesn't make it more likely for the attack to hit, it will just do more damage if it does.

I'm seeing people use HLC with Calculation and Recon spec on him. It's a very nasty setup.

just for clarification to the above set up. Bossk moves and takes a focus, Recon adds another focus. Bossk fires the HLC and gets 2 hits, a crit, and an eyeball. The crit switches to a hit because of the HLC, but the eyeball gets moved to a crit because of calculation. As it has been modified once, HLC doesn't switch it to a hit. defender rolls poorly and a hit and crit go through. Bossk can then change the crit to 2 hits so that the defender takes 3 instead of 2.

Yes?

and if you add Greedo to the mix, and you get through his shields, then one of the hits is treated as a crit? (face up damage)

Exactly.

Yes. I refuse to play Greedo. Everytime I've played him, he has done nothing but kill me, lol.

I ran Bossk with Greedo last night, he did some damage to Brobots but also bit into Bossk quite badly too! I'm going to leave him off next time I think at least until Scum get some better defence upgrades.