Here's what I'd do instead...
get you a good HOB overflow box (don't want to drill a stocked tank) and put that on the small end... then run you another 20long or 10gallon on the bottom and put equipment in there.. You can either do a chaeto fuge or a turf scrubber (what I personally use)
then run a u-tube return line and some power heads (if you're going peninsula style, you may need to run koralia 1s to get down to the other side of the tank with enough flow) and you're golden just make sure you run the rocks down the middle and keep them away from the glass so that you get good flow through the tank.
When you're stocking fish, its best to toss in the more peaceful ones first i.e. the firefish then the watchman/shrimp then the clowns. With the firefish, make sure you run a glass or screen top on the tank as they're very prone to jumping out. There's lots of other nano fish that can go in a 20long, I've kept alot of them in my old 20 long setup. Personally I like yashia gobies/candy stripe pistols much better than yellow watchmen.. they stay small and are less prone to eating crabs and small fish in the tank like some yellow watchmen have been known to do. Since you're relatively new, I'd keep the fish load down especially until you get a fuge or scrubber going to export nitrates and phosphates. I also recommend running a bag of carbon (chemipure works) and a bag of GFO (granular ferric oxide like rowaphos) to absorb phosphates.
I have ran a cpr aquafuge 2 with the skimmer and its crap. The dang inlet on the pump gets clogged easily, it never skims right, the back section is easy to trap detritus and pass microbubbles and doesn't flow right. Best to not waste the money.
I don't personally run a skimmer on my tanks anymore, I have enough liverock and the scrubber, carbon, GFO to take care of alot of stuff and my tanks aren't packed to the brim with fish either.
I do prefer the all snails approach to CUCs because crabs are serial killers and opportunists when they get big. Mixing 5 astreas, 1 turbo, 3 nerites, 3 ceriths, 6 nassarius, and a single tongan fighting conch will definitely get you going well in the CUC department.. you can always add more later if you need to.
I'll give you some ideas for really nice nano fish that I've kept/keep:
B&W/standard/picasso/na-ked/whatever ocellaris (probably the best suited clown for nanos)
yellow assessor basslets (expensive, but very colorful, hardy, and leave other fish alone)
black cap basslets (cool looking and cheap, may be semi-aggressive about territory though)
yashia gobies (my fave shrimp goby!!!)
neon gobies (yellow, blue, or "snarknose" varieties)
hector's and rainford's gobies (colorful and eat pods, add once the tank has been established for about 5-6months)
Citron gobies (they're hardy, eat well unlike yellow clown gobies and look good, cheap too)
yellow wrasses (get a lid on there, one of the more peaceful wrasses that can be put in smaller tanks)
firefish (I'm partial to purples and the expensive yet really attractive helfreich's)
golden nosed eviota gobies (very hardy, I didn't feed it for a month in my old biocube and it still stayed fat off pods lol)
trimma gobies (like an eviota, but different colors)
So there's definitely alot of cool nano fish out there, I don't like blennies personally as I had a VERY bad experience with a bicolor in my 20long and I don't care for sixline wrasses either as they can get aggressive too in a small tank.
stay away from dwarf angels, they're semi aggressive as is so why make them even more aggressive in a small tank.
best of luck, PM me if you have any specific nano fish questions and I can give you an opinion.