Client States: Canada is not a client state... nor is guam... both are too far out the poles of the spectrum.
Guam is a semi autnomous territory; essentially, it is equivalent to any backwater imperial world.... (I woudl point out that few imperial worlds are more integrated into the 3I than any US territory... No say in the big government, but subject to the big government none the less... but with some local autonomy).
Canada holds a place similar to either Arden or Daryen groups... big enough to be important, small enough to rely upon neighbors for joint efforts, and too independant to follow blindly thier "Big Neighbor".
No, the classic client states of the US are Taiwan, Israel, Panama, and until the 1930's, Cuba. Big enough to be known, but so small, that were it not for the threat of Big Brother, someone else would have taken them.
In the 1980's, Poland, Cuba, Czekoslovakia, Yugoslavia, and Romania were soviet client states. North Korea and Vietnam, they were (and still are, to a degree) Chienese Client States.
THe Germanies could be argued to have been client states of the US and USSR, but politically, neither was in lock step despite propaganda to the contrary.
A client state is one that needs the relationship, but for whatever reasons remains independant. In some cases (Panama, for example) taking over would not have been locally decried after as while, but would have triggered severe repercussions with other nations. Some client states are puppets, others are more like "Pets": favored so long as they don't bite the hand that feeds them. Small enoguh that the "Big Brother" can slap them into nonexistance should they become too unreliable. (Can we say "Iraq"? I knew you could!)
Most of those client states, IMTU, are just that... they are not imperial, not seeking to join, but are also unable to divorce themselves from the Imperium's economy (In some cases, the Hive Fed or the Zh. Consulate, or the 2K worlds, as appropriate). Politically, they are too dangerous to unite by taking over one at a time... and the ones important enough to bother with are in fact important enough that a change would be destabilizing, and possibly force a local organization. Which would either result in a "terrorist hotbed" or an independant autonomy.
Then again, IMTU, the Imperium wants that buffer of well behaved client states to make their warships being outside the Imperium seen to be normal for crews, too. So, when they have to react, neither the crews nor the populace get upset.
Guamanians. btw, are US citizens in all respects, except CIA paranoia. As are half of the samoans, half of the Virgin Islanders, all the Puerto Ricans, the Marshal Islanders, and until recently, some panamanians.