Wireless Access

As I understand, clients' device connects and moves to AP with the most powerful signal.I mean, if my device is connected to AP at -65 dBi, and there is another AP with -60 dBi signal at client, my device should move to anothere AP?

Maybe, or maybe not.

Ultimately, whatever is programmed into the client's driver determines what the client does.

So, the decision to move from one AP to another performs clients' device?
I need to create network to handle with video streaming, and if signal attenuation is more then -65-75 dBi it becomes issues with stream.

I think, that I have to place AP close to each other, but I need roaming at fixed signal attenuation!
How can I do it?

You can create that network, but if the client does not choose to roam, you will not meet your requirement.  You should contact the manufacturer of the client to see how you can have it roam the way you want it.