they are like the worst of the worst no matter their size and such...
No, that's not accurate.
Especially Druids are like "everything must be natural"-Hippies, right?
No, that's not accurate.
Openly siding with a Devil or making a pact with him, the equivalent in our own world be to say an ISIS member is one's best pal.

No, that's not accurate.
Tiefs are seen as low-class, or "trash Devils" by more pureblooded or higher class Devils right?
No, that's not accurate. Tieflings are not hellspawn. They have never been hellspawn. Their original entry in their very first official publication made no mention whatsoever of the hells or the abyss, and cast them as plane-touched. they have always been plane-touched.
And the Devils will not openly tolerate anyone deserting this... eternal War thingy, right?

No, that's not accurate.
If Tieflings see someone running around WITH A FULLBLOODED DEVIL, no matter how tiny, isnt that like really, really bad in terms for their location? ^^
No, it isn't, not necessarily. It may be, but not necessarily. Many arcane casters use imps or quasits as familiars, and have done for many, many years. It says nothing about their own alignment or goals.
with their super high base stats, maybe even 15 or even 20 and try to either kill or force every Tiefling back into the 9 Hells, or not!?
No, that's not accurate. Tieflings are not from the hells.