Well, the fireknives are an okay choice, but no longer optimal in the modern Tau codex for my money. I've never liked Helios suits.
Here's the thing: Back in the old days, dating back to the Tau's inception, you *had* to take three hardpoints on a suit. One of those hardpoints was a multitracker and the other two could either be a twin-linked weapon or two separate weapons. The only way to get multiple shots out of a Crisis Suit was to mix weapons. That's where 'Fireknife', 'Helios', 'Bladestorm', and the rest of them came from.
Fast forward to 6th Edition. No more three system minimum, no more multitracker taking up a slot, and now you can take two of the same weapon. What does this mean?
Stop mixing weapons!
Crisis Suits work best, to my mind, as specialists. They are designed for eliminating specific threats. Few of the weapons in the Tau arsenal compliment each other well enough to bother mixing.
Fireknives? Well, if you're taking them to take out Heavy Infantry, you may as well take double plasma guns. They can *still* double against light vehicles (deepstrike to hit rear armor, etc.) OR, take one fusion blaster in the team on that third slot to give yourself anti-vehicle efficiency.
If you're hunting vehicles, take double missile pod or double fusion blaster. If you aim to kill infantry (though this isn't really a Crisis suit's job), take double burst cannons.
To go economy-style, twin-link the weapons instead, but if you can scare up the points, double-up. Of note, twin-linked Missile Pods can be very handy against flyers, especially if you can get a single markerlight hit on the flyer.
As for that third slot, I recommend Flamers. Flamers + Supporting Fire = fabulous overwatch support unit. Keep your suits somewhere near other units that might be charged and watch the Wall of Death pay off.