I'm an RF guy, and the Rigol Calibration kit board looks like it has fifty ohm microstrip.
I don't know how the cal board is used, but I assume that all channels are set to 50 ohms to use it.
However, the combined impedance will be 10 ohms (five 50 ohms in parallel: see, RF ain't all black magic!) which won't match the central feed which I assume will go off to a signal generator presumeably with a 50 ohm characteristic impedance. Whether this is a problem or not I don't know, but it seems strange to go to the effort of making 50 ohm microstrip and then mismatching the feed point so grossly. But depending on the use case, if all channels are equally badly treated perhaps it doesn't matter.
It is possible there's some impedance matching such as a wideband transformer on the board to deal with the mismatch, but I couldn't see any evidence of this. You could use a minimum loss pad to provide a broadband match too (basically a pi or T made out of resistors, they're inherently broadband, but more lossy than a transformer).
A couple of other comments. Firstly, depending on use case, it may be necessary to terminate any unused channels with fifty ohm terminators or you'll get erroneous results. Secondly, the closeness of those BNCs on your board might make for a bit of fun when trying to fit the cables, you might want to do a fit check first, I did a similar thing with SMA connectors on a board a few years ago, you don't make that mistake twice!