Honestly it sounds like your 29g is fully stocked. In terms of tankmates:
IME clowns won't really work out, I've tried it, didn't go so well.
A manderin in a 29g long term is just not a good idea IME, unless you have a rather larg refugium hooked up to that to produce the nessecary amount of copepods. Even manderins that readily accept frozen food at one momnet have a habit of changing there minds and only accepting live pods IME.
I have kept seahorses with a manderin in a 44g with a 20g refugium. The tank had over 100 lbs of LR and a good amount of macro algaes. JME
If you were to do a pair of seahorses in a 29 gallon there are several options for fish you could keep, but you would have to keep the stocking density fairly low without a really good filtration system. You also couldn't keep more then a pair of seahorses and a couple other fish and maintain good water quality without some work IME.
You could keep many of the smaller gobies. Personally I would look at the twinspot as it tends to stay on the sand and will eat left over mysis. A clown goby, they are cute and don't take much room. A yashia although you hardly ever see them. A Tangora goby. A yellow or wheeler's watchmen (the pink spotted get to big and are kind of aggresive IME). Any of those would do well paired with a pistol shirmp.
A rainford goby is a looker and safe. A red headed catalina or catalina if you plan on keeping your tank in the low 70's (which I recomend)
In a tank that size most of the other sand sifters are out.
You could get away with a royal gramma basslet.
A pair of firefish will work, but mak sure the tank is enlosed as they jump.
A bi-color, midas, or tail spot blenny might also work out O.K.
Most of the other seahorse safe fish need more room then the 29g will allow.
Yes pistol shrimp are safe with seahorses. I find peppermint shrimp to be a preffered tankmate as they also have a taste for aiptasia.
HTH
cut and paste generic basic advice here
When asking for help, that really isn't the best attitude to take. We
are here helping people we don't even know to try to benefit the hobby. A search of the forum would have anwsered the question itself. After you have replied to the same question a hundred times or so, the advice does tend to get a bit generic.

Leaves us wondering why people ask the same question over and over and over again.
Not getting down on you, but be nice to the people you ask for help from. We're all doing are best.
