I only have (4) poles / DC reels that I currently use and I do a mix of Coastal Fishing / River and Lake Fishing.

When going to the river and lake I like to target Catfish and Crappie.

When I go to the Coast, I like to target Redfish and Flounder.

I have (2) poles set up for Catfish with 65 lb braid.

Is it safe to say I can use the #15 lb braid to target all (Fresh-Water Crappie and Red-Fish / Flounder on the coast using the same #15 braid that way I don't have to switch every-time I go to the coast?

I have a couple of rods I keep for crappie that I use on the coast too. My coastal needs are for trout and reds and are used on the flats throwing plastics.

No, I would not do this, I would not use braid on the coast (or frankly at all). I use 4lb mono when fishing for crappie and I spool my coastal rods with 8-10 lb mono, it’s more than enough for slot specs and reds.

buy some extra spools for your reels and just swap them out.

Yes 15lb braid is fine. Great for Reds and flounder especially if they be a bit larger. when using for Crappie I tie off a swivel and use a 6lb to 8lb mono leader. The swivel leader really helps in line twist. I fish for both too and this is how i use one reel for both. Best part is when in the wind fishing for Crappie light mono tends to get wrapped up in the reel as it blows back in from the wind. The 15lb braid for the most part will not.

I use a light braided line when salt water fishing. I like to use a heavier fluorocarbon leader, (20 to 25 lb) about 24 to 30 inches long to keep the braid from getting damaged by shell or the fish gill plate.

Sure. I don’t particularly like braid that light, it is so fine in diameter that I don’t like working with it, but if you like it then use it. I never switch out my line going from fresh to saltwater and saltwater to fresh. These days, I’m using 30# braid and fluorocarbon leaders. No swivels, Albright special knot to join the braid to fluorocarbon.

In fishing, there’s about a million ways to skin the same cat. Some people like nylon monofilament, some like braid. They can all work fresh or salt, you just have to go with what makes sense to you.

No need to be rigid about fishing, but there are plenty of rigid fishermen. Perfectionists, too, but they mostly gravitate towards fly fishing.

I personally like 20lb braid on my crappie setups just because of the little bit larger line diameter and ease of working with. That should be more than enough for fishing on the coast as long as you use a Fluoro leader like mentioned above. Braid doesn't like anything abrasive (ie oyster beds or bridge piling with barnacles)