These men are the guardians of the Temple of Oaths on the Phalanx (the great flagship of the Imperial Fists). Naturally, these guys are also the ones that will go on to form the core of the Black Templars in 40k, along with Sigismund.
Hence, they are the zealots, the ones who might otherwise fit in very well with the Word Bearers. And they're also armed to the teeth.
The best way to think about these guys are that they are bloodletter analogues. Yes - I know that's slightly heretical to compare them to daemons. But hear me out. They all have power swords, and hence will be able to slice and dice their way through power armour. But its a bit better than that. They also have ranged weapons (bolt pistols) and both types of grenades. Better yet, they all have artificer armour. So even if they don't have a 5+ invulnerable save, their armour is simply superior to all around them. Additionally, the character for the unit has 2 wounds and everyone has an extra pip in WS, just for fun (well, it ties in well with their strict training regime to be fair). They are clearly kitted out for close combat duty, and should be used appropriately.
Not many, to be honest. The worst thing is that they are not all chosen warriors. Hence its only the squad character that will be issuing and accepting challenges. This isn't a big negative though. And secondly, its necessary to realise that they are a melee squad -- they're not meant for ranged pistol shooting (even if they get a bonus to hit due to the Imperial Fists Legion Rules).
But remember -- they're intrinsically slow, and will in all likelihood need transportation of some kind to get them where they need to be.
Below are a number of builds to think about.
5 Templar Brethren, legion vexilla, Chapter Champion with thunder hammer (195 points)
This is something of a base line unit. It should ideally be used with a rhino, drop pod, or other type of transportation.
5 Templar Brethren, legion vexilla, 2 plasma pistols, Chapter Champion with plasma pistol (230 points)
Similar to the above, but with a small host of plasma pistols since the AP they provide is arguably superior to shooting at BS5 with bolt pistols. Again, they need transport.
10 Templar Brethren, legion vexilla, all with combat shields and melta bombs, Chapter Champion with thunder hammer (385 points)
This is one to marry together with the legion's rite of war (hence the shields) and transport around in something like a land raider. They're dangerous to everything and can back it up. Add plasma pistols and a nuncio Vox to taste.
This unit is a solid choice for the Imperial Fists player. Although pricey, it must be compared to parallel units such as the Emperor's Children Palatine Blade Squad in some ways. To be clear: the Palatines are slightly cheaper, but the Templars are kitted out differently and can be boosted in an Imperial Fist army that is using the legion's rite of war. They're also really good for adding in a close combat orientated HQ selection as their personal guard.