Flexi and Summa works together really fine, my company - as other Summa Dealers too - sell both together often for years.
Contour Cutting works really fine, but there are some Limitations with specific features:
OPOS XY: Functionality to avoid problems because of bowing (non-linear deviation of the print along the Y-Axis) is not supported with Flexi 10 until now, will be released with the next version. I don't kno if the "Contour Cutting without RIP" function will support that in the future, but there you can set it up manually.
OPOS Barcode: Functionality for Batch Processing of Contour jobs (plotter will scan a printed Barcode of each job and will load the according jobs from the computer) arent available now, and when I asked for shortly they told me is not planned. SAi Pixelblaster does it, as Onyx Postershop/Productionhouse too, or as far as I know Caldera ... but these are no plotting softwares.
Saving different Setups for the Plotter in the software is limited, its ok for Speed and pressure, but e. g. parameters for Flexcut don't (like Onyx RIP does) ... Flexcut can only be activated in Flexi.