My Fiends of Slannesh(chaos spawn) are on the calvary bases they came with. Is this going to be a problem duing a tournament or should I just switch them over to monster bases?

I'd switch them over to monster bases. They should be on 40mm x 40mm monster bases.

The fiend of slaanesh models are old and from the previous edition rules, that's why they came with the smaller bases. I have two of them, so I know what you're talking about. I put mine on 40mm x 40mm bases. ;)

Thanks, thats what I figured.