Brought some used floating rotors, seller said have been used twice and Seems true as they look like new, however once fitted they are out of true and cannot set brakes up without a fair bit of drag. Can I still straighten out floating rotors with a adjustable spanner?

Yes

I’ve always used molgrips clamped around a rag. Borrowed a friends disk truer. It looks nicer but absolutely no better.

Yes you can. I’ ve bent mine in crashes and straightened them out again

Same as any other.

Thanks for replies, I wasn’t sure if it was the same due to the aluminium spider