[FRex]

There is also Interslavic that continues from Old Church Slavonic: http://steen.free.fr/interslavic/

I natively know Polish and I know some very little Russian back from high school and Interslavic is more clear to me than Slovio.

I'm interested in linguistics so during one CS conference in Zagreb I asked a few Czechs, Croats, Slovaks, Slovenians, one Belarusian (native in Russian actually but he knew some Belarusian) and back in Poland a few Poles who know only Polish and the results were quite mixed.

It really might be better to just treat this as a curio and speak your own (especially in your own West/East/South Slavic language subgroup) and hope for the best. Or just speak English...