So why i cant make board like for tablet so lcd is connected directly to board ? - often you may need to make and adapter where on one side is connector supported by your board and on the other side of the adapter is conenctor supported by the display (sometimes you need to reorder pins, sometimes the connectors are completely different and not compatible). If your on board connector is 100% compatible with display cable / connector, then you can connect them directly.
You can use any display you like and is compatible with your board. However, if you are planning some future with your product (e.g. future production), be sure, that you will be able to order that same display again even after year ... or two .. or five ....
This is what we use with iMX6Rex module:
http://www.imx6rex.com/application/accessories/