Were Stark and Baraggan Arrancar before joining Aizen? In both Baraggan's and Stark's flashbacks, they look the same as in present.

Did they become Arrancar on their own? However, if they did then how could Aizen promise Baraggan more power?
We have to disambiguate normal Hollow from Arrancar.  Normal Hollow don't retain much in the way of a human-like form and are relatively mindless beasts consumed with the hunger for powerful souls.  Arrancar evolve Soul Reaper-like powers and remove their mask, allowing for a human-like appearance.  The higher in rank the Arrancar, the more human-like they appear.

It is perfectly possible for a Hollow to evolve on their own into an Arrancar of varying rank.  That is to say, there are natural Arrancar in Hueco Mundo who were at a specific power level before Aizen approached them.  Several of Aizen's court were at Adjuchas and Vasto Lorde levels before he arrived.

In the case of Barragan Louisenbairn and Coyote Starrk, not only were they already Arrancar, but given that they are the Secundo and Primero Espada respectively, it is more than likely that they were already Vasto Lorde.