I’ve taken a look at Ogone and it looks like a relatively straightforward payment gateway. It should be easy enough to create a payment module for ogone that would plug in to the KonaKart module architecture. There are source code examples to follow and a tutorial for doing this in the User Guide.
I don’t know too much about onetrail but it seems to be a gateway product of some kind for transforming / communicating data between systems. KonaKart is ideal for integration projects due to the customization hooks and extensive APIs.. So, whilst I don't know exactly what you want to do with onetrail, I would expect you to have no problems doing so.
For icecat... yes you could integrate products from their catalog easily enough.. take a look at
http://www.mediasanto.com/ which uses KonaKart - and may well use icecat as the source of its products but I'm not certain.
--Heidi