Those clowns (loaches?) are going to get to be 12 inches... I know, I know; they grow slowly. I don't care! They still shouldn't be in a 28 gallon tank. LOTS of people keep them in that size tank but I realy feel strongly that you should never keep a fish in a tank that will not accomodate it at adulthood and that applies to people who 'plan to upgrade' as well. Sometimes things will happen that will keep you from aquiring a suitable home and I realy have to beg you to either buy those loaches a huge tank now or give them away. It just isn't fair to keep a fish with that sort of growing potential in a small tank. I know most people would argue but I've been seeing a lot of this lately and though I'm sure there are many responsible owners out there who will get a bigger tank when necessary, there is also a chance that something unpredictable will happen. You cannot forsee the future. Now, having come to the end of my rant (sorry people), I might answer your question
First off, unless both your swordtails are female, you should get another female. If they are both male, they will fight so get several more females or get rid of one male. Obviously, the number of swordtails you add/remove will affect the number of new fish you can add.
Some cories, a school of cherry barbs, danios, smallish rasboras. A dwarf gourami or pair; or a single (female prefferably) blue/three-spot or pearl gourami or a bristlenose pleco. Only add ONE of these options though as you may find yourself over-stocked otherwise.
Unfortunately, unless you give the exact size of your loaches, it's difficult to say how much more you can add... based on their adult sizes... well you are WAY, way... WAY overstocked but so are so many other fish keepers so never mind
Still... I urge you to remove those loaches to another tank to provide you with more stocking options. There are plenty of smaller loaches you could get instead and the clowns would thrive in a larger tank and grow to their full, remarkable potential.