Runing conditions will depend entirely on the max engine it's transom rated for. For full fun go for the max engine, probably 25/30 HP. If so, with an 18 expect much less performance and fun.
I don't know about the rib cat specifically, but other inflatable catamarans (zap cats/thundercats) are great fun and amazing in the rough stuff too. An 18hp on a 380 will be significantly underpowered though. It would be ok with one person but will most likely struggle with two. If you're after ultimate performance and handling save your money and get a proper thundercat hull with a 50hp.
It would go and handle well with an 18 they are very stable , its a very very wet ride that thats gets extremely wet if you add more than 2 people to it or run into any wind / tide . Fun but wetsuite on for every trip