Probably a red sex link mix. The white tail is what I am basing this on. Red sex links have the dominant white gene that covers black coloring but not red. However, red sex link cocks are mostly white, which is why I think it must be a mix.
Thai game fowl of some kind seems right to me, but I am far from an expert on Asian game fowl.
I would try to find different stock to breed these with because they may be siblings. If you get unrelated stock you could line breed them. The rooster especially is beautiful.
You could also cross...
When they are really young it's not as much of an issue, but at 5 months, yes, they are going to go after the newbies. You can just put them in there to see how it goes. Sometimes the unexpected happens.
If you do that, it seems to help to do it at night, and put the new ones on the roost bars...
I did a google search on Thai game fowl, and AI gave a brief explanation, but it seems a lot more complicated than the information they have on the topic.
I did find some pictures, but to me the different types all look pretty similar. I'll post a few of the pictures I found.
Basically, I...
Even though the photo is taken in shade, I can see a little shininess in the shoulder feathers. I think. If so, that's male specific feathering. I do think this is a late blooming cockerel. In addition to the other things already mentioned, the patchy color pattern looks male to me.
Let us know...
I agree that better photos would help. I'm guessing it is a very late developing cockerel. Mounting other hens, especially at a young age, would be uncommon for a female.
The black one with the rose comb is 100% male, since he has male specific feathering coming in.
The blue one with the single comb is kind of in between. By comparison to the other, I also think it could be female, but it is fairly well developed in the comb, too.
I really think the answer...
I think females based on their crests, but it's pretty early for Polish. Watch the wattles. Color or growth in that area before 12 weeks or so probably means a male.