Circle Casting and Spell Slots

I have players using circle casting for a Aura of Vitality spell - Prolong version.

We are bogged down a bit with some of the wording as some of them state 

"Each secondary caster contributing to the spell must expend a spell slot (no action required). If the spell fails, these spell slots aren’t expended."

But others state they need a spell of equal or greater level.

If a player wants to circle cast a spell of 3rd level, unless stated otherwise, can a secondary caster use a 1st level spell slot to circle cast a spell of greater level?

If it doesn't say the spell slot has to be of a specific level, then the spell slot doesn't have to be of a specific level.

The primary caster versus the secondary casters.

The primary cast has to actually cast the spell you are enhancing with Circle magic, so he must use a slot appropriate to the base spell.

The secondary casters have to follow not only the base Circle Magic rules (30 ft, take the magic action, be a spellcaster/Pact magic), but the specific rules for the particular add on effect they are creating, which vary a lot.

Some of the Augments do not ever require the secondary casters to expend a spell slot (Distribute for example)  

Expand (increases Area of effect) requires any spell slot, can even be a 1st level.  Cantrips do not qualify.

Supplant (reduces needed Material cost) however requires a spell slot equal to or greater than the Primary spell cast. 

So it depends on the chosen Circle Casting Option.