I looking to buy a st croix avid spinning rod. I'm probably gonna get a 6'6. I'm asking what power I should get. I Kinda want to use the rod for just about everything perch, catfish, largemouth, and even striped bass if possible. The biggest striped bass I cych is about 30 inches. So what power should I get and what reel should I match it with?

I'd step up to the 7' and get a medium heavy, give you a better overall performance

The Avids are nice rods and pretty powerful for their labeled specs. A 7' MH / F is pretty versatile, but you can get away with a M/F in the Avid if the cover is moderately heavy. 

Fast action is the best overall action out there (spinning or casting), in my opinion.

The Avids are nice rods and pretty powerful for their labeled specs. A 7' MH / F is pretty versatile, but you can get away with a M/F in the Avid if the cover is moderately heavy. 

 

I agree with this. If you have a chance to go into a shop and feel the MH/F and the M/F, do it and see which one you like better. But the MH might have a little more backbone than you want/need.

I have the 7'0" M/F spinning, and it's about perfect for me.

Your asking one rod to do quite a bit with a range from panfish to stripers. Your best bet is to get either a 6'6" M/F or a 7' M/F. It might be a little over rated for the perch and slightly under rated for the catfish and stripers, but you will be able to make it work. Personally I'd go with the 7' rod to give you a little extra distance, especially with the light baits. If you feel more comfortable with a 6'6" though that'll do it for you. 

M/F in whatever length you want is going to be the way to go. It's going to be under powered for stripers and overpowered for panfish but that's probably the best you can do if you're trying to get away with just 1 rod. 

I'd go medi9um power as well, and up the length.  More length gives you more rod under load with a fish on, and helps keep bigger fish pinned when they do weird things. Think of it as a chock absorber.  A 7-6 rod has more travel than a 6' rod.  The extra 18" actually equates to 36" of rod travel when fighting a fish.