There is a misconception in the basis of your question here: common gender is not the same as not specifying gender. The common gender is an actual gender, used in some languages (Danish, Swedish, Norwegian Bokmål, Hittite, etc.). I would not advise using that to refer to non-gendered forms.
In languages (or pronouns, or any other context) that does not specify gender at all, the most common practice is to simply leave out the gender-designating letter altogether. So where 1ms is ‘first person singular, masculine’, the most common way of writing ‘first person singular, any gender’ is simply 1s.
The same logic is sometimes applied to number as well; for example, the Lithuanian verb is always the same in the third person, whether singular or plural, and this is often noted as simply 3, with no extra letters at all. The same can be done for you in English.
If you wish to be more specific and reduce the risk of misunderstanding—a single number in running text is not necessarily very clear—you can include all the possible values instead of leaving them all out. In that way, the Lithuanian verb forms would be 3spl (or 3s/pl for enhanced readability), and you in English is 2splmf (or 2s/pl-m/f for readability). This is the approach usually taken when a particular form is relevant to multiple (but not all) cases in a given paradigm, like the Sanskrit ending -bhyām, which is idadu (instrumental, dative, and ablative dual).