15 to 18 gal is cutting it pretty close,
I have a 20 gal, not much larger that has been running now 2 yrs. & it has 5 fish, none that reach over 3" in length however, a VERY critical factor and over 26 corals.
A lot of keeping everything happy in a tank of this size is starting with healthy juveniles that only grow to a 3" max. & how/when they are introduced. I first introduced the 2 baby Ocellaris Clowns, after a month came a baby Royal Gamma, 30 days later came the baby watchman goby, finally the banggai cardnal fish which was full size from a friends tank, i didn't expect to actually add another but it needed a home bad. Everything gets along fine long as all are well fed 2X a day.
And the corals get their food from what ever is left in the water column & doing fine on that & lighting.
So it can be done with no issues as long as you run a skimmer & are diligent about weekly 10% water changes but if my tank was any smaller at all id drop 2 fish and id have never taken the Cardinal fish, down to just 3 definatly with coral & fish growth factored in down the rd.
If i ever loose a fish from age etc..i probably wouldn't replace it in order to lower the load on a tank with so much coral just to stay safe or else remove some corals to just my faves.
Larger tanks are obviously without a doubt better always for all reasons but yes, it can be a bit crowded & still a very mellow happy tank as long as your very careful on what & when its introduced as well as careful live rock placement for good flow & places to hide & swim thru for the fish and oh so importaint weekly water changes................