Q. Can I buy cat-food that has chicken and fish oil in it? What about a mixture of beef and fish, is that dangerous for animals as well as people?

A. Talmud Pesachim (76b) mentions that one may not cook meat and fish together since this mixture is considered to be dangerous.
Shulchan Aruch (Y.D. 116: 2-3) explains that it can be a factor in severe illness as leprosy. Magen Avrohom (O.H. 173: 1) debates whether this danger still prevails in our days. However, our tradition is still to avoid eating them together.

Pischei Teshuvo (ibid. 2) quotes Shebus Yaakov (2: 102) that the above also applies to meat from fowls also.

However, it seems obvious that it applies only to humans, since other prohibitions of Sakana (danger) also only apply to humans
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Rabbi A. Bartfeld as revised by Horav Shlomo Miller, Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch Ehrentreu Shlit’a