Well armoured models can be a real pain for opponent’s in the early campaign, and indeed in one off, non-campaign games.
In short, they’re far more apt to win battles of attrition, as relatively few gangs have much access to effective armour.
Honestly? The best advice I can give is to just go wild. Field whatever you want, get a feel for the game and where your tactical preference lies. Don’t worry about efficiency or anything like that. I actually find the game gnarlier and more fun when I just cobbled together a gang from whatever models I’ve built.
To my mind, it better reflects the not brilliantly funded militia aspect of the game. It’s more of a challenge to make whatever you’ve got work, and I for one find that fun (other opinions are available, nobody is playing the game wrong!)
Though I would say the game really comes into its own when you’ve got at least half a dozen players in a campaign. You’re pretty much guaranteed a solid mix of gang types, especially if each player’s selection is kept secret until the first game.
But as I said, these are just my preferences, and absolutely nobody is playing the game wrong.