Tuan,
Dynamic Chain Diversity (DCS) allows the AP to dynamically alter the number of transmit chains it will
use to respond to client devices. The number of chains used is based on the number of chains supported by the client at the time of the association. Each chain will use just one of the antennas and each chain requires its own antenna. These chains are your spatial streams (MIMO).
If a client is only capable of 1 or 2 spatial streams, then the AP will only ever transmit to the client using 1 or 2 spatial streams. But....the AP can use *all* of it's antennas (chains) when it's listening/receiving a transmission from that client. The AP having more 'ears' allows it to hear the client better.
Unless you have a special reason, you want this function to remain enabled.
Regarding having the same model antenna for all connections - technically, there's nothing stopping you from doing what you're saying - and it may even appear to work.
But, the results are going to be completely un-predictable. Trying to explain this would require going into quite a bit more 802.11/RF detail, so I'll just leave it at - unless you don't have any other choice, don't do this.