If I am not mistake to use flex track then I could not use Kato, Would have to use Peco Does peco make flex track or have to mix in Flex track from Atlas and will they work together?
You are not mistaken. Flex track and Kato turnouts are not designed to just plug into each other. While it is technically possible to adapt Kato turnouts and connect them to flex track. Kato turnouts are really designed to work only with Kato track. Both Kato turnouts, and Kato track pieces, are manufactured with a permanently-attached gray plastic "roadbed" piece under each piece of track. Peco, Atlas, and Micro Engineering, turnouts & track, do not have this roadbed piece attached.
The sensible plan would be to use either Kato track & Kato turnouts, OR use Peco turnouts, and whatever brand of flex track you want. Yes, Peco does make flex track., so if you prefer non-roadbed type track, you could use Peco turnouts and Peco flex track. However, plain track, (the kind without the plastic roadbed), is typically compatible from brand to brand. So you can easily mix & match brands.
As an example, Peco flex track is not commonly carried by many hobby shops, but Atlas flex track is. To get Peco flex track, you would need to have the shop special order it for you, or order it yourself online. Peco flex is also more expensive than Atlas flex track. So, if you wanted, you could use Peco turnouts with Atlas flex track. (Many have done just that.) That way you would have decent flex track, at a lower price, and excellent turnouts, a winning combination.
The only thing necessary for connecting one brand of plain (non-roadbed) track ,to another brand of turnout, or track, is to have the same "rail code" on the two track brands you want to connect. So what is rail code? It is simply the height of the rails in thousandths of an inch. For instance, two popular track types/codes commonly used in HO-scale are "Code 100" and "Code 83." The Code 100 rails are 100/1000ths" high and the Code 83 rails are 83/1000ths" high. Either code track will work fine. The Code 83 track just looks a little more realistic, in terms of rail size. The rail codes are all about appearance, not function.
So, if you decide to use Peco turnouts, they are available in either code 100, or code 83. Track, (whether Peco or Atlas), is also available in either code 100, or code 83. Just pick one code, and use track, and turnouts, that all have the same code.
While it is possible to mate two pieces of track with different rail codes, you're just starting out, so I recommend keeping things simple, by just using one code.
Good Luck & Have Fun;
Traction Fan 🙂