Sorry to say this, but Butterfree isn't a Purple Pokemon :/ According to the Pokedex, he is white! You can find lists of Pokemon by colour on various websites, notably Bulbapedia. The fully evolved Gen 1 purple Pokemon are; Arbok, Nidoking, Golbat, Venomoth, Muk, Cloyster, Gengar, Weezing, Starmie, Aerodactyl and Ditto. Any of these Pokemon would be fine!
In reply to your questions, yes, you may evolve your Koffing. (See Rule 4)
You can hack in your Golem at the start of the game at a low level, or you can hack in a Geodude and level him up to a point, then hack evolve him, whatever suits you really. (See Rule 5) For the other two Pokemon you are considering, I believe they are both obtainable in the FireRed games? Therefore, you may not hack them in, rather you must catch them normally!
Pokemon 6 HAS to be the very first Pokemon you find on the route, similar to a Nuzlocke, it's just the luck of the draw really! Even if it is an Ekans, you just have to run with two Arboks. (See Rule 9)
Before I can sign you up, I need you to confirm your selections for Pokemon 4 & 5.