Most species of Angler would do fine in a 20gal. tank. However, just a single Whisper filter is not going to be adequate. They are messy fish, yet require almost pristine water conditions. They are very intolerant of nitrates, ammonia, etc. You'd at least need a skimmer on the tank, to support an Angler.
Mantis, while having a bad rep of breaking glass, are quite interesting pets. I've kept a few, over the years and they've been my favorite little critters. They'll actually sit and stare at you, follow you and try to intimidate you. The main reason they break glass is because they're trying to burrow...they get to the bottom of the tank, think it's rock and give it a whack.
You can solve that problem by laying down a piece of acrylic into the bottom of the tank.
Just as Anglers, they are too messy eaters. A Whisper filter is do-able, but you'd have to religiously keep up with your water changes, etc.