Tyranids don't get Transport vehicles. Neither do Daemons. And Chaos Space Marines don't get to have 6-man vehicles. So it doesn't come up. And, Armoured Fist squads no longer exist in the way that you're thinking of.
So, I understand your ignorance. That's alright. It's not bad. You just don't know. And, I've only just come up with this idea yesterday/today.
From the current Big Black Book (unless you've got AoBR, in which case it's the small rule book), page 67;
"The only limitation of a dedicated transport is that when it is deployed it can only carry the unit it was selected with [...]. After the game begins it can then transport any friendly infantry unit, subject to transport capacity and other special exclusions [...]."
Nowhere does it say that the vehicle must carry it's 'payload' before - or - at deployment. If it did, there'd be a lot of angry Tau players.
And, even if that were the case (which it isn't, to clear confusion), Codecies supersede the BBB. And Codex: Space Marines along with Space Wolves (possibly Dark Angels, but I've lost the Codex , and I don't have Black Templars), allow ten-man squads to take Razorbacks.
Most people just haven't used this tactic because of the rule of Combat Squads in Space Marines, allowing you to split the squad into five. Fully enabling the Razorback to use it's transport capacity again.
Although, with the advent of Codex: Space Wolves,
One, The Combat Squads rule is gone.
Two, Space Wolves can't have Heavy Weapons in their 'Tactical Squads'.
Ergo, the ten-man 'Razorback Squad' will need to get used if you want more Heavy Weapons. And, the difference between a Rhino and a Razorback is only five points. More if you upgrade. Which I would. TL Lascannons are sweet.
Codex Witch Hunters still has the 'wrong' wording. Since it's old. And Sisters' Squads with more than 6 models can't take Immolators at all.
And Inquisitors with a large Retinue can't ride in their vehicles, etc.