Archie was enever a paedophile.
Those people have actual feelings of lust and/or love towards a child that has not yet hit puberty, in the same way "normal" people feel such things towards people more or less their own age.
For Archie, it had to do with control and punishment. He could make his daughter do certain things and then keep quiet. He was in every aspect of her life, the only male figure close to her. She was more like property rather than a kid he felt affection for. It was also a way of hurting Glenda, as she felt pushed aside for a younger model, like she said herself. So he may have used Ronnie as means to punish Glenda for whatever she was supposed to have done now.
After it turned out Ronnie did in fact, have a boyfriend and slept with him, Archie was no longer in control. He was in his daughters home-life, but could not be in her school-life, and was not the only person in her sex-life either. So she was dirty and that is why he took the kid from her.
It would have been very well possible that he had started on Roxy as well, but she always doted on her father, unlike Ronnie, so he knew he could control Roxy far more easily, by just giving her attention and presents. To control Ronnie, he needed the abuse, as she did not respond to being daddy's little princess as well as Roxy did.
Derek is just as controlling as Archie. So I feel there is a real risk he might at one point commit a rape, the victim could be just about anyone. He could fly into a rage over Carol or Alice sleeping with someone and attack them. He may use it to have power over someone else, be it a family member (like Lauren or Abi) or not (it would not have surprised me if he had overstepped that border with Lucy). Derek does not care about other people's feelings, wants control of every single person and situation, fully expecting everyone to do as he says. If they don't he reacts with violence.
If Derek is Kat's lover, I could see a situation where he wants to resume/continue the relationship, but she doesn't and prefers to remain faithfull to Alfie for once. Derek may force himself upon her and it all comes out, because Kat just can't keep this a secret (let's just hope she isn't pregnant).
Same for Tanya, I can totally see Derek hitting on her while drunk, she turns him down and he won't take no for an answer. If such a thing happens, especially with Tanya, a whodunnit would be huge, half the Square would be a suspect, especially if Derek does not manage to actually go through with it, because one of his brothers walks in, both of them know and love someone who has been raped, so their reaction would be even more severe.