Well, you can click a button and have a panic puma to counter T70, however there isn't really anything allied side that would do the same, short of AEC, which does arrive pretty early, but since brits are irrelevant, not sure if its worth mentioning.
251, being light vehicle, doesn't really need offensive vet, especially when its flamers are extremely strong from get go.
Vet gives it mainly mobility, but that mobility does make a massive difference for it, which we have seen yesterday multiple times.
At least it needs more then 2 seconds now to kill fully healed garrison now, but other then that, thanks to how flamers and their accuracy works, 251 is a retreat wipe machine if you commit to a chase.
Guards or AT Tommies? FHT is high risk, high reward unit. I don't see any issues with it. Yes it comes early but its basically relegated to covering flanks as the game progresses. T70 much better bang for buck and its ability to scale.