The loop made with the nail knots works well in fact I use it. Over the top I put some electrical type silicone tubing which shrinks when heated to seal the knots.
I then tie a bimini in the backing and use a loop to loop connection. Make the bimini loop at least a foot long so that the reel can pass through it.
With regard to the length of leader. I use a maximum of 9ft when using large flies. Any longer makes it difficult to turn the line over.
You should be OK with a 9wt set up for tarpon up to 50lbs but anything larger may give you problems.
I have caught tarpon up to 70lbs in Cayo Largo in Cuba at that time of year.