Lol, well, since we're here in the Saltwater Beginner's forum, I'll assume we're talking SW. I don't know the Flex tank but after a quick look on Amazon, I'm assuming it's the one that's bowed from top to bottom? In that case, I think the filtration compartment could be customized to work well for SW. However, if we're talking about a nano reef tank, you should know the light will have to be replaced... and, of course, reef lighting can cost a pretty penny.
If however, you'll do a FOWLR (which is a great start and will give you time to research the best reef setup for you) then you can still have some cool inverts and a small fish or two without a large investment in lighting. If it's the tank I think it is, I see that you lose some of the overall volume and space to the rear sump, so please keep in mind that many saltwater fish require a territory they can call their own and may defend it... to the death, lol. No, but seriously, I would stock this tank very carefully and plan for that stock from the very beginning with the rockscape. If you will have two fish, you really should consider the space they will use and plan to offer them both comfort with your design. For example, if you really want a clown and another fish, that other fish should be a bottom dweller so the clown can have the usable swimming space to himself. The rockscape should provide a little cave or overhang where a sand dweller can have a burrow. If upkeep and filtration are well managed, I think you might get away with a percula clown, a pistol goby and a pistol shrimp in this tank. I have a yellow watchman goby and a candy cane pistol shrimp and their relationship is fun to watch... plus, the goby is such a character, particularly with his big frown.
Can I just say... firefish have a bad rap, and they are quite timid, but are also beautiful and don't need a ton of side to side swimming room. I kept a firefish alone in our 20 gallon long reef for more than a year and he hung out front and center every day for hours... seemingly unafraid of me or the goings-on of small children right outside his tank. He was a really great fish to have but sadly couldn't handle new additions to his little kingdom. If I had a Flex tank, I think I'd go FOWLR and dedicate it to a and some cool inverts.
Also, before you decide on SW, please take the water source into consideration. Some people get away with it but it's always a risk to do a salty tank sourced on tap water. RODI water is where the guarantee of safety is and while a RODI system can be a big purchase, well, you may not need one... at first. RODI and premixed saltwater can be purchased from most LFS's. This is not a bad idea for a small tank but please remember that you'll need saltwater for water changes and RODI water for topping off. There's also the LFS's trustworthiness to consider. It's not unheard of for an LFS to not keep up with RODI system maintenance and still sell water that may be full of nasties that may give you an algae garden or worse. So, while buying good water is an option, it comes with its risks, and the more time and money you invest in a tank, the less risks you'll want to take. In other words, LFS water may not be great, but it will likely be good enough for a FOWLR... but when and if you start hurting your wallet with corals, who are much more sensitive than your average fish... well, at that point it feels a whole lot safer to make your own water so you know exactly what's in it. Portable RODI systems with quick connect/easy to use options can be had for less than a Benjamin.
Sounds like a 15 gallon bowfront which would be interesting. @ the OP, the thing to keep in mind when starting a saltwater tank is that length=options for fish and bowfronts tend to be pretty cramped for width which can increase territorial issues/aggression among fish. I started with a 36 gallon bowfront and had to remove a number of fish from my tank due to aggression issues (most of which were ones that were supposedly peaceful). Personally I wouldn't do anything smaller than a 20 gallon long to start with as it gives you . The 15 gallon bowfront will be limited to 3 small fish MAX. You could possibly do a pair of Ocellaris Clownfish in there but they can become very territorial once established. Ultimately though you go with what you got and what makes sense to your space and budget.
Starfish is out for a 15 gallon tank. They require pristine water parameters and get too big. About the smallest one available is the Tiled Starfish which gets 5 inches and it would definitely starve to death in a 15 gallon tank with just 2 fish. As far as peaceful hermit crabs, the best of the bunch are the smaller red hermits. They tend to do the job without bothering too much. Some people still have issues with them, particularly in smaller in tanks. The blue legged hermits tend to be a bit more aggressive and trouble prone than the Reds. You could try the Dwarf Blue Legged as they stay small enough that they shouldn't be too big of a problem: