Can I use SSD Samsung 850 EVo , 250gb in Macbook Mid-2010 (Macbook 7.1) without mounting screws?
You can run your Mac with the SSD unmounted, but it is HIGHLY DISCOURAGED. This is because:


The connection can come lose, which will cause kernel panics and data loss
Could also harm the SSD if it bounces around in the case
Movement could cause damage to internal components, such as connections.


The best thing to do is to mount the drive. Post a picture of the mounting screws, and someone here will point you to the right screwdriver. They're almost always cheap, and it's worth the money.