They should be able to understand.
Term `Serbo-Croatian` is politically invented term. Serbian language belongs in group of Stokavian Slavic speech. Croatian language belongs to Kajkavian speech.
What confuses you is that is Serbian language speaking language of Croatian state and Croats but, Croatia don`t have intention to tell it openly. Due to large influx of Serbs when Ottomans invaded, in regions of what is solely ethnic Croatian territories- Zagreb, Krizevci and Varazdin and mixing of two kin Slavic peoples, ethnic Croats were assimilated into the Serbs. But it was welcomed by the Croatian elite. Just later, due to German and Austrian influence (who became sponsors of Croatian magnates), Croatian elite started to manipulate and to label Serbian language (culture, heritage) as Croatian language. One could even have understanding for this if just Croatian elite (instead to tries to restore Croatian identity on original Croatian ethnic space) didn`t start to speak of Serbian territories and people as of Croat property. So, Croatian elite didn`t speak only of regions (its approximately 20% of today`s continental Croatia; if you look at map region is in a upper left corner near to Hungarian and Slovenian border) Zagreb, Krizevci and Varazdin as of Croatia but they also started to grabbing Dalmatia, Slavonia (region; not same as Slovenia or Slovakia), Lika, Banija, Kordun, Dubrovnik, etc... including even Bosnia and Herzegovina.
All in all, if you want to see how sound Croatian (Kajkavian) language go travel somewhere around Krizevci. There you can hear it. i think that today less then 3.000 of people in Croatia speak Croatian language. Official and speaking language in Croatia is Serbian language.