Arcanine, Omastar and Venusaur as defenders

Arcanine maybe has the time to use a FT, but no time for a FB vs. a high level Vaporeon.
Omastar- wg is to weak, vape is medium defender due to HP, which Oma does not have to much. I don't have examples, did not encountered any as defender.
Venusaur - if my memory serves he gets beaten by a half CP Goldbat 3-4 times if you dodge. I guess a high lvl Crobat beats him without revives for as many fights as needed to take the gym down. Or any of the strong fire attakers that are now needed only for blissey(fire spin ones for example).

And BTW, Arcanine featured with FF and Venu with Razor. Bite and Vine were subpar also back then,

Arcanine with Snarl Wild Charge might be semi-viable in weaker areas, in that people will charge in expecting fire attacks, probably using some water attacker, and the wild charge might screw them up. I'm not sure what you meant by Bite-FF (as both bite and FF are quick moves)? You mean Flame Thrower? But in any event, someone will probably march into that with something resistant to fire anyway and walk all over him.

Omastar doesn't seem very good, frankly. With RS/RT it's an interesting mon, but I don't even find them useful for prestiging, there are better performing pokemon if you need water prestigers.

Venusaur isn't a very good defender - it's more for attacking water types. And yours has one of the worst defensive movesets too. No good. Maybe as attacker, but PB is quite a bit worse than SB.

Venusaur with vine wipe is a great pokemon for attacking vaporeon gyms destroys them fast, as a defender it needs razor leaf to hold its ground
If your looking for a good niche defender with decent cp try Jolteon with volt switch
most of the defenders you listed are still good but they need A rated moveset

Caught a high level Snarl/WC Arcanine a while ago, mostly leave it in gyms, don't know how it performs but once I prestiged against a Crunch one and it was a nightmare :D