Hi @Eugenijus and welcome to the Meraki Community. The polarization is a characteristic of the physical antennas themselves, so that is not something that can be configured or adjusted. The setting in Dashboard allows you to change the antenna type, depending on what is actually connected to the AP, such as the ANT-20 omnis or an ANT-25 patch for example.
The ANT-20 omni has vertical polarization and the radiation pattern is the typical "donut" shape, so if the AP is mounted on a wall or pole and those antennas have vertical orientation, then your "donut" is omnidirectional in the horizontal direction. Kind of like this:
If the APs are now mounted flat to a ceiling, then your "donut" pattern is turned sideways, and much of the RF signal is being propagated up/down and sideways as opposed to all around like in the image above. There isn't a way to adjust that, the antenna polarization is fixed and part of the antenna.
Depending on the coverage area and number of APs, this may or may not work ok, you will have to test and take measurements. If you associate to any SSID and bring up the local status page by pointing a browser to ap.meraki.com you will see your connection and signal strength and can walk around and do your own basic mini site survey. Depending on ceiling height and distance, it may end up working much better with ANT-25 patch antennas which have articulating arms and can be aimed at the coverage areas.
On the other question, yes, if you are not going to use the 2.4GHz ports on an MR74 or MR76 and the 2.4GHz radios are going to be disabled, you don't need antennas on those ports and the 2.4GHz ports need to be protected with any 3rd party weatherproof "N" type dust cap. The plastic caps that come with the AP are just for protection during shipping, not for weatherproofing. Note that it it is an MR84 or MR86, those are 4x4 APs and all 4 ports are dual band and will always be in use. Sounds like you have 74/76 so that's fine to disable the 2.4GHz radios and not have antennas on the 2.4GHz ports.
More info in the antenna FAQ as well.
https://documentation.meraki.com/MR/Other_Topics/Frequently_Asked_Questions_regarding_Cisco_Meraki_A...
Hope that helps!