Hi, welcome to the forum. Glad you joined.
First you do not have a 14 week old rooster, you will have an immature cockerel. There is a world of difference in how a mature rooster interacts with mature hens than how an immature cockerel interacts. There probably would not be much drama with a mature rooster, there could be a lot with an immature cockerel.
A mature rooster would almost certainly move in and take charge. There is no telling what the drama with a cockerel will look like and what drama there is could and probably will change as he matures. It might start out similar to integrating a pullet, especially if his hormones haven't hit yet. It could involve him trying to dominate the young pullets. He may try to mate them, that's not really about sex but how he establishes domination. He may totally leave the younger pullets alone. He may try to mate the older hens to try to dominate them. If he tries that they may just run away but they may beat the crap out of him. If he tries that the dominant hen may knock him off when he tries or she may beat him up. It could actually be pretty peaceful, but the odds of some drama at some point are pretty high. I'd think at first he will probably want to hang out with your younger pullets.
How much room you have is important. The more room you have the more likely no one gets hurt. The tighter they are packed the more likely you are to have issues.
I don't know how big your frizzle rooster is, often they are bantams. That might affect how he interacts with those full-sized hens. It could make things more peaceful than otherwise, it could put him in some danger from the mature hens. The way I'd approach it is to integrate him like you would any other chicken but observe and see if you think you need to intervene. Each chicken has its own personality and each flock has its own dynamics based on the individual personalities, how much room they have, and other things. This could go unbelievably peacefully or you may need to intervene. When you are dealing with living animals you never know.
What will happen if he mates the hens? Nothing if you don't incubate any eggs. But you probably knew that.
The frizzle gene is dominant but it is a lethal gene. If you get a frizzle gene at both sides of that gene pair then the chick dies. In the future you might not want to mate a frizzle with a frizzle since that really hurts your hatch rate. Since he lived and is frizzle, he has one frizzle gene at that gene pair and one not frizzle gene. If you mate him with non-frizzle hens then about half the chicks will be frizzle, half will not. The fatal part of that gene will not have an effect.
I wish you luck. Odds are you will work it out, just observe.
