Yes, this switch should work with that device.
No, this switch will not connect to the Pro Tac HLX.
Unfortunately, this product has been discontinued and we no longer have that information available. We recommend reaching out to the manufacturer for this inquiry. Their contact information can be found here: https://www.unitytactical.com/contact-us/.
Yes, it will. It will come with a picatinny rail attachment and the HK416 has a picatinny handguard.
Yes, it will work with both of those units.
Yes, it will work with Steiner lasers. Here is a list of most of the lights and lasers this pressure switch is compatible with:
Lights:
Surefire M300, M300U, M300V, M600AA, M600AA-DSS, M600U, M600V, M600V-AA, M620, M620V, M720, M952, M952V, P2X-Fury, X200, X300, X300UA, X300UB
Lasers:
ATPIAL-C, CQBL-1, DBAL-A2, DBAL-A3, DBAL-D2, DBAL-I2, DIAL-100, ITAL. LA-5/5C, MAWL-DA, OTAL, AN/PEQ-15, AN/PEQ15a, AN/PEQ-16A, AN/PEQ-16B, AN/PEQ-2A, RAPTOR ES, RAPTOR Lite