Looking to get a reel mostly for large northern pike, but ok for musky too.  Is 5:9:1 too slow of a retrieval rate?

It'll work fine for general purpose. Less resistance at the handle and easier days casting.

I'm admittedly new to musky fishing but 5-8 hrs burning 6.3 and 7.1 reels and big baits was a huge adjustment for me over bass fishing.  

I just did a big thread on this in a fishing forum. You have given one of the least important factors possible for this question. First, what type of reel, and specifically the spool size? You need to know the diameter of your spool. As a general rule, low profile bait casters are going to be much slower than a spinning reel because their spools are much smaller. Inches per turn, or cm per turn elsewhere is what you want to know. For example, a Daiwa 300 is often considered a decent "lower price" (it's still really expensive) musky reel. With the 6.2:1 gear ratio, it is rated to pull in 28.8" per turn of the handle. In reality it will be slightly less than this since you probably won't fill the spool with line 100% the diameter of the spool. Call it 27"-28" for that reel. Lets compare to an old school reel, a Penn 710 I recently tested. With a 3.6:1 gear ratio, that reel pulled in 26" of line per turn, that was actually tested by me. With the 4.1:1 gear ratio, that same reel pulled in 29.5" of line. So we have two reels with WILDLY different gear ratios that are running similar speeds. So keep that in mind. A Spinning reel with 5:1 ratio is not slow, it's probably faster than 95% of the baitcasters. A round baitcaster is somewhere in the middle. Most low profile baitcasters need the higher gear ratios to match the speed. Again, its the spool diameter, specifically the line diameter on the spool that matters.

I'm a big time northern pike fisherman. Even so, it's not the fish that directly determines the reel you want, it's the bait and how you want to fish. The only time I've found where you want as much speed as you can get is topwater fishing, which I don't do a ton of. Regular reels work fine too, you just have to crank faster on the hook set. For the majority of casting like cranks, swimbaits, spinner baits, spoons, etc. it's not important. Reels that pull in 20"-30" of line per turn work fine, it's hard to tell the difference. The high speed craze is mostly bass fisherman. They are doing all kinds of goofy things, and a lot of the time they want those fast reels for no other reason than they can pull the fish up out of the water/cover sooner and just yank them in. That's not really an issue with pike. Even in thick weeds, it's not like you can rip a 36" pike to the surface like you see guys do to 15" bass. I also like to troll and baitfish, and gear ratio there isn't a big issue either. I will say my favorite trolling reels are pretty slow, and that extra torque is nice for pulling in big cranks and planer boards and such. Another factor that allows you to apply more torque to the reel with no change to gears is to just put a longer handle on it too. All my trolling reels have big power handles. So in my opinion, gear ratio, and reel speed in general is way down the list of important factors in a pike reel.