Shields?

the other option instead of shields would be PD similar to the gil-es anti tacmissiledeffense .. so lets say you had a unit that fires a beam targeting missiles bombs and or artillery shells
but everything else goes through because its too fast to hit (gameplaywise it just does not target those)

What about Shields that
A: Absorb straight up damage
B: take a particular number of hits(not a standard health pool, just a float for number of hits taken/can be taken), then collapse for a time (cooldown & recharge) which then re-gen at full str?

This combination would slow slow heavy hitting units, but allow light mothballs that you chuck at your enemy to get through, for ex:
Bolos/ants take 2 seconds to reload @ 84 damage a shot (48DPS)
Dox fire 2 shots a second @ 10 damage a shot (20 DPS)

My example shield has 20 hits that it can take,
One Dox punches through in 10 seconds with 200 damage mitigated
One Bolo/ant punches through in 40 seconds, with 1680 damage mitigated (holy crap that's a lot)

Two dox punch through in 5 seconds with 200 damage Mitigated
Two Bolos/ants punch through in 20 seconds with 1680 damage mitigated

Four dox punch through in 2.5 seconds with 200 damage Mitigated
Four Bolos/ants punch though in 10 seconds with 1680 damage mitigated

or another possibility (potentially T2 variant of the above?)

A: Absorb straight up damage
B: Shield Emitter concept, similar to the above, taking x shots, but the number of shots is on a building by building basis (within the emitters radius)(think protoss plasma shields around their buildings)
Ex:

My example shielded building has 20 hits that it can take,
One Dox punches through in 10 seconds with 200 damage mitigated per building
One Bolo/ant punches through in 40 seconds, with 1680 damage mitigated per building (holy crap that's a lot)
(time for tier 2 **** my pants damage mitigation)
Slammer ( 100 damage @ 4 shots per second, 400 DPS)
One slammer punches through in 5 seconds, with 2000 damage/building mitigated (here's where I'd **** my pants if i didn't do the math that follows)
Leveler ( 600 dmg/shot 2 shots/second 1200 dps)
one Leveler punches though in 10 seconds, but 12000 damage is mitigated PER BUILDING (what have I done!?!?!)

Div the time in half for all units if doubling up in units, damage absorbed remains the same

so you mean your shields would stack? no thx

Even worse. It would mean that shields could be cheated around by including 1-2 Dox in your army - only to reach the projectile limit of the shield.

No, shields must only offer procentual damage reduction, no sponge like mechanic. But even that must not happen unconditionally. If the threat level is too low, no damage reduction must be applied.

How do you reduce the spacing?

You'll need to edit the file <PA install dir>/stable/media/pa/units/land/land_barrier/land_barrier.json
Change the field "area_build_separation" from 0.5 to something smaller.