Specifically the old "Big figures" (pre-Colossals like IC Sentinel and HT Sinestro)... I know you can choose to use their rules as written as to pushing (that you can use their "they can take an action when they have 2 tokens and take pushing damage as normal" instead of Colossal Stamina) so you can give them an action every turn without taking damage if you give them WP/Indom in any way, but what about the rest?

In this case, I'm wondering if you can opt to use their version of multi-attack as written. As it's written (and with the DDM in place now), the Sentinel/Sinestro could make 2 ranged attacks without using any powers and the second gets -1 Dmg (which would compound with the DDM if I'm not mistaken to make it -2 Dmg), but current Multi-Attack rules wouldn't allow him to make 2 ranged attacks unless one of them was using an attack power.
In most cases, this wouldn't make enough of a difference, but all 3 "Big Figures" have clicks without a damaging attack power on it and that extra attack of 1 damage might come in handy at random times (when the new Multi-Attack won't let you attack again if they are both ranged).

So can I opt to use the Multi-Attack (or any other part of their power sheet) as written?

To the best of my knowledge that is not the case. I could be mistaken, but as I recall the only things I remember being ruled upon in such a way were for the figures whose effects allowed them to not take the damage.
IE, a guy whose rules stated something like "they can take an action when they have 2 tokens take no pushing damage" would not be damaged (and it always bothered me that the rules were never updated to reflect this ruling), but I don't recall the reverse being true.
(EDIT: It is definitely important to note that Colossal Stamina does not deal pushing damage.)
Multi-Attack as defined on the PAC is the Multi-Attack used across the board. The only one who gets around this is the original Galactus as he doesn't have Multi-Attack. Rather he has Cosmic Multi-Attack.

True, I just wasn't sure if I could use what's written in their rules because it doesn't highlight it as Multi-Attack (being a heading), it's just under the attacking rules and it says "Sentinel [or Sinestro] has a special Multi-Attack that allows him to attack 2 targets with 2 separate attacks..." (which seems more like a reference than a title). I hadn't heard a ruling allowing this so I thought I'd ask.