10 lb should be fine in flouro for flatties. You might lose the chance for some bycatch such as whiting and bream which are more shy. The lighter the better normally but flatties have some sharp gill rakers which can destroy leader.

I use 4-6 lb and get snapped off 3-4 times out of 50 odd flatties when they catch the line under the gill raker. If you want to use light line keep them submerged until the last moment, when they start head shaking at the surface is the most vulnerable time to lose them. If I want to keep a flatty I normally walk it onto the beach with a little wave or wash and they stay pretty calm.

 if i use a 6lb leader would that work as well?

As Champ mentioned 10lb flouro is fine for flatties & he already mentioned the reduced chance of bycatch. 6lb flouro is what I use for flatties & bream & I find it works fine. Keep the flatties head underwater by holding the rod tip low during the fight like Champ mentioned, that will reduce the chances of it sawing it's way through the line. I find a fairly long leader of about 5ft to pick up more fish for me than a shorter leader of say 3ft. Soft plastics can get hammered really quickly by the blowies, especieally when fished slowly, so take lots of tails & be prepared to go through a pack or two in a couple of hours. I like to use suspending hardbodys & make sure they are digging thier way along the bottom kicking up puffs of sand etc. Retrieve as slow as possible with plenty of darts & pauses. I like to twitch the lure during the pause so that it just kicks around a little on the spot, that has undone plenty of flatties for me & they usually take it just as the retrieve is started again.

Can I ask why you think you have the wrong set up? Is it just a feeling or is it lack of success, if so what lures & what areas are you trying?

 Really the reason i ask is, i have not been getting any success at all haha.. i fished for a long time with my dad then stopped for quite a few years ever since he passed away, only just now have i started fishing again and back then, it was only me and my dad and basically it was all just put some line in the water and then hope for the best. But i was never really taught anything about picking lines, leaders, etc2.

Im new to fishing in Perth and I have been trying the canning bridge, mt henry and the jetty near como.. but pretty much all i have caught is one bream, a couple of crabs down at como and countless blowies. I have been using prawns, mulies, gulp minnows. 

Como Jetty area isn't much chop for most fish IME though there are often schools of whiting in the shallows at night & the occasional mulloway if you put in the hours out the end. Canning Bridge is blowie heaven in a river that is blowie paradise. There's plenty of good fish to be caught there but you have to wade through the blowies to get them so that usually means night fishing will give you the best results. I've never fished Mt Henry Bridge but I know there are some decent bream lurking around there. I'd also give the Narrows Bridge, Point Walter & Bicton Baths a go. If transport is not an issue then Claremont Flats, Point Roe, the Old Traffic Bridge & Deepwater Point are also good areas to fish with Claremont Jetty & the Narrows being my favourites. Further up the Swan around Bassendean & Guildford is great for targeting Bream & there are only a few (if any) blowies up there so working soft plastics slowly is very effective. There are tailor almost anywhere from the city down to freo at the moment so you should be able to get into those if you throw some lures or baitcast whitebait anywhere from just before sundown to about 10pm. Personally I'd leave the soft plastics at home unless you are happy to fish at night when there's less blowies around or go far enough up the river that thier population thins out. Plastics work great but unfortunately the blowies will make very short work of a pack of tails unless you work the lure quicker & IME that cuts down the number of fish you will catch dramatically. Hardbody lures are the go for flathead & bream anywhere the blowies are & if you choose suspending models you will be able to work them really slowly & effectively. I find that if you want to fish during the day then targeting bream & flathead on hardbody lures is the way to go otherwise I restrict nearly al of my fishing to night time when the blowies are not as prevalent. If you want to use baits then night is definitely the go. Try prawns, whitebait, half mulies & whole baby mulies. Hope that helps a little.