Oh heavens no, every ex-feeder rat I have ever met has been just wonderful, and they're absolutely NOT harder to tame than a rat born to an ethical breeder. The only difference is instead of having their baby days be cushy and spoiled, they had terrible living conditions, hunger, maybe pain, and lots of fear - they can smell when snakes are close by. Imagine having a mortal enemy always living right next door to you, and you have no idea when the door between you might just open...
What I really mean is, sometimes it takes just a bit longer to get them to truly trust you, to place their lives in your hands, to know without question that you stand for everything like love, caring, food, fuzzy hammocks and lots of veggies and treats. But to be honest, the first time a rat trusts you is the most precious moment you two can have together, and if you have to work hard for that moment... it only makes it sweeter.
Now when you're first handling them you do want to be careful - you won't know just how scared they are of you until you've seen them for a while and interacted with them a bit. But if you keep your movements slow, your voice gentle, and just keep feeding them treats from your hands, they'll realize it's not the Big Grabby Hand of Doom, but the Big Gentle Hand That Bears Yummies. Soon you'll have six devoted little followers who can't get enough of you.
Ask as many questions as you have need - we're here to help. I think all of us have rescued rats, many have ex-feeder rats (remember Carina Snake-Biter, Shelagh?), so we have lots of tips and tricks to share to help make your road to rattie bliss as smooth as possible.