Yes, this light will fit just fine.
The Streamlight Compact Lighting System light will take up about 6 picatinny slots and the remote switch will take up about 8 slots.
To mount the Streamlight Compact Lighting System to an MLOK handguard you will need MLOK to picatinny adapters like the ones at the link below.
https://www.opticsplanet.com/trybe-defense-magnite-m-lok-picatinny-rail-section-3-slots.html
Applying continuous pressure to the switch will activate the lights momentary on feature. So as soon as you release the switch the light will turn off. A simple tap of the switch will activate the constant on function.
The Streamlight TLR Compact Lighting System TLR RM1 tail cap is not interchangeable with the TLR-7A.
The Streamlight Compact Lighting System may fit a Walther PPQ M2 but is designed to mount on a picatinny rail on long guns. The Streamlight TLR-7A would be a better option, you can find it here:
https://www.opticsplanet.com/streamlight-tlr-7a-flex-tactical-weapon-light.html
You can get the Streamlight Compact Lighting System with the pressure switch, you just have to select that model on the product page.
Yes, it should work just fine.
This light will attach to a picatinny mount.
You can use this light on a rifle just fine.