Question: Can I apply a formula to an intercompany account or will that cause issues to the elimination process?

Answer:  Yes this is possible. 

• For example, within the budget and forecast process, some clients want their planners to input only the receivable or payable with the system automating the offsetting entry in the partner entity. This can be achieved using a member formula or business rule. 
• Generally, the calculations would be written to the base entities/local currencies. With regard to intercompany accounts and the elimination process, OneStream writes to C#Elimination.  

Source: Office Hours 2020-7-2 - Partner Enablement