All Kaeltars have a pistol, Flash Pulse (with a fantastic WIP:14) and Electric Pulse. Profiles chose between a light shotgun or combi rifle. The Kaeltars aren't going to be winning most gunfights but in a link they can make use of good burst.
Electric Pulse acts light a defensive tool, as does flash pulse, but other than that they are line troopers.
With electric pulse you can walk Kaletars into melee as part of a coordinated order along with a more deadly melee specialist and force your opponent to chose between an automatic stun or taking the hit from a melee specialist (or dodging which means no offensive threat for you.
The Kaeltars will go down to most fire, but be sure to find out what their weapons are like first, and where possible use defensive modifiers. Their high WIP combined with Flash Pulse can make them a pain.
Kaeltars are worth bringing in any list in which you have 2 or more Symbiont armor models. While the temptation to take 2 Symbio-Mate profiles will be strong (save up to four guys from fire for one turn each, bring two models with CoC), consider bringing the bombs as well. The Zone of Control attacks of the Symbio-Bombs are very deadly, and create a rare defensive ZoC tool for missions where you need to protect a Designated Target or keep your opponent out. Additionally models with high WIP such as Neema Saatar can use the attacks provided with much more reliability.
Keep them out of harms way placing at least one model with CoC out of harms way, unless you already have a well hidden/not obvious Lieutenant.
Active Turn Role
Kaeltars will usually be supporting a Triad, but can be used to score objectives if necessary. The light shotgun model can also be used to win face to faces or clear bunched models if conditions are right.
Unless facing heavy modifiers the Kaeltar wants to flash pulse. Usually you will be looking at base 17 to hit (before enemy modifiers) but even with a -6 you are still better off shooting than dodging, and will often do so with 2 burst. When facing heavy penalties you can consider a dodge. Don't forget about electric pulse. Engaging into melee with an opponent then pulsing them can create a difficult situation for the enemy, especially if you can get more than one model to engage.