The T-rex and Repashy diets are the same--I think T-rex is just what they manufacture in small bottles to sell in pet stores, whereas Repashy is what is manufactured in larger bags. I don't think they even make the gargoyle diet anymore. I would just go with the crested gecko diet as it has been further developed and a new and better version of it just came out (better taste and better growth rates with the new diet). You can buy it here: http://www.pangeareptile.com/store/i...ategory_id=165. Plus if you get both a garg and a crested you can feed them both the same thing.
Do not mix the diet with anything except water. It is complete as is, and mixing it throws off the balance. If you want to feed fruit as a treat that's fine, and you could mash it up and put some CGD in it, but that would just be a once in a while thing--not a staple food.
You don't have to feed insects at all--it's up to you. You can get crickets at most retail pet stores. Phoenix worms can usually be found online--I'm not sure where the best place would be to buy in small amounts because I've only ever bought them in bulk (http://www.phoenixworm.com/servlet/t...rms/Categories).
Cresteds are somewhat social and females usually like to be kept in groups. Males cannot be kept together as they will fight. Female gargs usually get along, but they can be more aggressive toward eachother so you have to be careful in pairing them up. I would not mix cresteds and gargs in the same enclosure.