I have a 6’6” Medium GX2.

I plan on using it for bass and walleye. 
I like senkos for bass. I also have rapalas I want to try out. Is 30# braid a good choice? 
Can I change the leader depending on what I use?  Or should I buy two rods one for deeper water, and one for top water? 

Two or more combos would be better. It will make you more versatile.

Fishing rods are tools. You usually need more than one tool for a job. I mean, you dont just have one size screw driver to try to remove many different sized screws. 

The more rod and reels you have the better you can handle a situation.

Nothing wrong with putting on braid and then a fluro leader.  For senkos I like 15 pound hi viz Power Pro and 8lb leader.  I like the Alberto knot to connect the two.  But usually when I go out, I bring about 5-7 rods.  It depends on your situation and style.  Less is more for some people.  For me, if I am on my boat or kayak, it's nice to have everything I need in arms reach.  When I go wading or shore fishing, it's usually 2 rods max

You can certainly use that rod to fish a both senkos and crankbaits. Those rods naturally have a slower parabolic action, so I'd use something like 30 pound braid with a 12-15 pound leader of mono/fluoro/hybrid. Using braid as your main line will help with hooksets on single hook techniques (like the senko). If you were just using it for crankbaits, then 12 pound hybrid/mono/fluoro would work great.