You're off to a great start on your Fluval build!
I have the M60 that has been running for just about a year-and-a-half. They're nice tanks, but I also did some modifications to the filtration area, water pumps and a custom screen top. The recommendation for adding the ceramic media for extra filtration is a good one. I have a bag of the ceramic rings that helps with added biological filtration.
Are you planning on using the bulkhead for water changes? I ask because I do not; I like to siphon out any algae and detritus that gets in my rock. But I did utilize the bulkhead for my Auto Top Off; which I strongly suggest implementing on any tank.
Good call on adding the Gyre pump. I added an Ecotech MP10 for extra flow as the single return doesn't get enough circulation in the tank.
For your aquascaping, try keeping the rock away from the back panel an inch or two to allow water flow to keep the backside free from detritus build up. I only have three pieces of live rock in my tank that are stacked away from the back and the circulation keeps that area very clean. Starting with less rock makes the tank look empty but as corals are added and begin to grow out, these tanks fill up quickly.
As for fish suggestions, you have a good list.
I really like blennies and gobies.
Any of the small Ecsenius spp. blennies are perfect. I think the Starry Blenny is a good choice as long as it behaves itself. I had one that was a monster and it killed a couple of other fish, so adding it last might help against further aggressions. The Starry Blenny will mostly hang out amongst the rock work mostly perching on the top; gobies are better suited for the bottom of the tank.
A goby and pistol shrimp pair is very enjoyable to watch. There are plenty of nice looking gobies that form a relationship with pistol shrimp, so you have lots of choices there. I'm planning on adding a Rainford's Goby soon; as they are great at picking algae on the rock.
I have a pair of Bangaii Cardinalfish, so your pair of Pajama Cardinals will do great.
Other suggestions could be one of the smaller flasher wrasses. They will add some bright color and they're active swimmers; your list for now has less active swimmers. I'd stay away from a Six-line Wrasse; although small and cute, they can become aggressive towards new tank mates.
Other fish for color include the basslets, dottybacks and hawkfish (if you don't plan on any shrimp.)
I added a very small Lemonpeel Angelfish to my tank, but within a year it was too aggressive for such a small tank. They're such beautiful additions but eventually become the dominant fish in the tank.
Keep us posted of your progress.
Good luck!
@jlanger, thanks for the great information. I know I have an ATO set up, but I never thought to tun it through my drain. I'll have to see if I can make that change (considering that I have water in the tank which make make it tough).
Love the fish you have going on...I may have to do some more considerations.