What do you mean by casual? If you think you can go there with your LFR knowledge and your LFR dps then, in fact, no. Flex is meant for organized groups that are not able to raid on a set schedule each week and can't bring the best gear, but still want to have a bit of fun and maybe a challenge. And this challenge is where you need to switch your casual thing off, because it can destroy you and your fun with a group that only fulfills the bare minimum for Flex, or not even that. You can PUG it, yes, but truth is you shouldn't.