Any research or context (business case, roles, personas, workflows, etc.) on which to base your design assumptions?
I've got no argument whatsoever with consistency and standards, but...
Net/Net: We all want a customized UI (the Fewest Clicks to Satisfaction)
You've already indicated that a given customer is likely to explore your products in a 'silo-ed' fashion, which lends itself to such a customized approach, rather than one-size-fits-all.
Aside from the UI issues, you also have the opportunity to collaborate with marketing folks in crafting an appropriate pathway, effective message, and compelling buying experience for each customer silo. Cross-selling to your other product & services might be part of that.
I'm assuming here that a login page is not simply a couple of fields and a button. There should be some consistency and best practices as regards raw functionality, of course. But if one product is used by technicians, another by salespeople, and a third by management, then you'll be more effective when you recognize that up front and craft your design accordingly.