Typical tejano music instruments include accordion, bajo sexto, bass guitar, electric guitar, drums, trumpet, trombone, saxophone, keyboards, piano, and of course vocals.
There are 3 different types of tejano music distinguished primarily by the instruments used, so yes the instruments in tejano music are extremely important. And they're aren't really any specific people associated with each instrument because tejano is very laid back and versatile.
Conjunto is the original form of Tejano music, this type uses electric guitar originally, and the bass and the drums were eventually added in. Conjunto's golden age was in the late 1940's and '50s.
Banda is the second major form of Tejano. This type added in the accoridian, more vocals, and bajo sexto. Later on in history the Banda people started incorporating brass- oriented instrumentation which consists of trumpets and saxophones.
Tejano's third type of music is grupo, and it began in the 1960's. This form included the keyboard and embraced multiple tendencies from the original form of conjunto with a few twists. Grupo also neglected the horn, saxophone, and the accoridian.