PVS14 is a head mount device. I have a Larue QD mount on mine that fits their QD riser for EOTech sights on an M4 and that works pretty well, but I almost never use it. What I use instead are IR laser/illuminators on a few rifles. For an M4 firing 5.56, it's good out to ~300m easy (or however you zero your rifle such that your trajectory intersects the line of sight twice). This is using .7mW lasers.
As for the EOTech, it works fairly well that way, but it's limited. You can only see in the dark when aiming. Or you can go back and forth if you choose. I just think it has a very limited use. I've never seen the mono-loc, but a guy on here used two hose clamps and a piece of radiator tube it looked like, and he was shooting (under 600m I think) using a 5.56 bolt gun with a 14 attached to a Vortex scope, and he was hitting his targets. But as was mentioned above, it's not made for it and it's not as accurate due to optics (?).
I imagine you have to zero the day scope to the 14 when you attach it. I'm guessing this based on my experience as an SDM instructor. We taught to use the lasers and head mounted 14's to 300m (most PEQ2A's were locked, and some guys still had PAC4C's --that's why I know .7mW will work out to there). But I farted around trying to get the 14 behind the ACOG (while head mounted still) and though you could see your target, aimed well, getting consistent hits was extremely difficult. I never bothered mounting the 14 behind the ACOG because then you'd of course have to move it back and re-zero it for day use.
For a rifle, for serious shooting at night using a day scope that you also use during the day, you can't beat a front mounted clip on. Provided all your scopes are mounted at 1.5" from top of rail to center of optic, and you have enough rail in front of the scope and both scope and NV device are on the same plane, it'll work on any of those within a given magnification range depending on the device used (some are good up to 12x or so). The housings are also built to withstand more abuse and recoil vs. the 14.
I'd DEFINITELY save up for the clip on. It's what I'm going to do in fact. It's worth it in the long run.
As for putting a 14 on a .308... You may get away with it for a while, or you may just get away with it period. Without an accelerometer to mount to the rifle though, you have no idea of the rearward acceleration of the NV device under recoil, and that's more important than the actual felt recoil of the rifle itself.