Concurrent running shouldn’t be a problem. Each zone has its own individual triac, which supports 800mA continuous and several amps inrush current. As long as your sprinkler transformer has a sufficient current rating, I see no problem turning on 6 zones at the same time.
180 zones is beyond the software limit of the microcontroller-based OpenSprinkler (honestly I am quite impressed that you have 180 zones…) However, OpenSprinkler Pi does not have any software limit on the number of zones.
Regarding concurrent vs sequential running modes, currently the only option is to globally switch between the two (i.e. either all stations run sequentially, or all stations are allowed to run concurrently). There is currently no support for subsets of sequential groups.