May one place a pan of fish next to a pot of soup on a hot blech before shabbos so it keeps warm for the seudah. Is this a issur of cooking meat and fish together?
This is fine and does not constitute cooking fish and meat together.
The prohibition of cooking fish and meat together only applies where the taste of the fish and meat become mixed up with one another.
In the case of two pots and pans there is no mixing up of taste (taste does not transfer from pot to pot) and therefore there is no prohibition.