For arguements sake lets say Grand Tournament standard rules
Unless I've missed something, GT standard rules are two thousand points and up to three detachments.
It would probably be pushing it to run more than one gargantuan squiggoth in your list unless your opponent has a similarly equipped list.
Why? Outside of having a decent toughness and a lot of wounds a gargantuan squiggoth is quite a point sink (450-500). And combined with only having a +4 save, a lot of multi wound dealing weapons will be punching through.
There's no explicit rule about it but including LOW units is generally something I try to agree or disagree with my opponents beforehand.
What about if someone wants to field a knight army? Disagreeing kinda means no game
How do you think I can best put this into an army list, that could be with minimal tweaking acceptable for most events. Are multiple LoW acceptable?
Well you have access to something in the ballpark of a dozen detachments to choose from. The most common one will be the battalion, requiring you to take two HQ's and three troops and giving you access to up to three heavy support choices.
In the case of this army the spearhead detachment is also a good choice, requiring only a single HQ and at least three heavy support choices.
However neither detachment allows you to take lord of war units, in this case the gargantuan squiggoth. For that you would need to take either a super heavy detachment or a super heavy auxiliary detachment.
I am at square 1 regarding army composition building, and would love to here your thoughts
Assuming your looking to stay away from most vehicles, a spearhead and super heavy aux detachment are great options. In order to try and maximize command points, you would need a battalion and super heavy detachment; but each gargantuan squiggoth runs you over 400 points and you would need 3 so while its possible to make a list its not going to do that well.