I'm specing my character to a total anti-legion guy, Does anyone have any tips for invading the fort island and taking the platinum chip by force?

Theres a place called the Gun Runners near Outer Vegas. Stock up on Anti-Material rifle and ammo. As well as handguns (assuming you built a gun centric character). 
 Also carry atleast one Incinerator with tons of ammo. 
 

I went in there and killed everyone (except the kids because they are apparently gods)

You can still go in the Fort and pretend to help Caesar,  to get the chip and go to the weather station inside the Fort and Talk to Mr. House, or reprogram the security bots for the Wild Card Quest, and then walk out.  I actually did this yesterday, I was also playing as anti- legion .

Go in there with lily and the dog and maybe they'll glitch out and start fighting the legion. The legion won't fight you, just them. A good way to thin the numbers if you're bored.

I just finished doing this. With a good sneak check, you can keep your holdout weapons. Buuut, you can also smuggle a handfull of other small weapons on ede or rex as well. Like C4, grenades, 44magnums, and sawn off shotgun:). Sneak and plant the C4 where it will do its worst, watch out for pretorians with ballistic fists.

I attacked his Fort with Boone at my side, and when all was quiet and still (read: dead) I posed Caesar's corpse and shoved a coffee mug up his ass. I then proceeded to activate a couple of Securitrons.

I also approve of using an Incinerator + Anti-materiel Rifle, works like a charm =)

Hmmm I was feeling completely badass.
So I surrendered all my guns, but kept Moria (9mm unique) as a holdout weapon.
Went inside Ceasars tent alone and pissed him off.
Then proceeded to fuck him up and his entire honor guard with a 9mm
Power armor and plenty of stims helped too (altho helmet got broken in the fight)