Prerequisite requirments
I don't know if it's 100% necessary for computational cosmology but I would highly recommend studying General Relativity if you can, especially if the course includes some cosmology as well (fairly common for undergraduate and graduate courses in GR). 

Fluency in differential geometry is (I think) an essential skill for a cosmologist, computational or otherwise. Studying GR and cosmology will also greatly help your understanding of the bigger picture, for example, the FLRW background, the Friedmann and continuity equations etc. 

Plus GR is a really beautiful theory! I would recommend that any physicist study it if they have the chance.