I don't know where whoever told you 500kg got their information from, but my data shows that even the smallest engined Roomster (the 1.2) can tow 700kg. This goes up to 1200kg for the 1.9TDI. It all depends what engine you were thinking about.
Kerbweight, and hence also the weight ratio, won't be an issue because all versions of the Roomster are heavier than the Kia Rio you are also looking at and that was OK.
Permissible towload is very important because this determines what you can legally tow. To tow a Sterckeman 390CP legally you would need at least a 1.4 litre Roomster.