There are no other forms of "somos" because it is already a conjugated form.
Here's a breakdown of the different forms of "ser" that you might be thinking of:
* Present Indicative:
* soy (I am)
* eres (you are)
* es (he/she/it is)
* somos (we are)
* sois (you are)
* son (they are)
* Other Tenses and Moods:
* Past: era, eras, fue, fuimos, fueron, etc.
* Future: seré, serás, será, seremos, serán, etc.
* Subjunctive: sea, seas, sea, seamos, seáis, sean, etc.
* Conditional: sería, serías, sería, seríamos, seríais, serían, etc.
* Imperative: sé (be), sed (be)
Let me know if you have any other questions about Spanish grammar!