Ulticlassing Psion & Profieicencies

If you multiclass into Psion after first level, do you get the proficiencies listed in your archetype? There is no language like that listed in the cleric that specifies that you only get the various bonuses if you take the class at first level.

Yes as they are a function of the Archetype, not the class itself.

Per my understanding of 5e rules:

If the proficiencies are gained “when you take first level” (like in most classes), then you only get them if you gained your first level in that class and would not get any more if you multiclassed into another class (i.e. I took 1 level of fighter and gained proficiencies in Str and Con Saves, then took 1 level of Rogue you would NOT gain proficiencies in Int and Dex because Rogue wasn’t taken at 1st level.

Now, more to what you are saying, if you gain the proficiencies through a CLASS ABILITY (like, in this case, a chosen Psion Archetype), you would gain those as they are in inherent to the class. Another example of this is if you took that 1 level of Fighter and 1 level of Rogue, you would still gain Sneak Attack as it is a Class Feature, not something gained at 1st level specifically.