Can we apply kernel seperately without upgrading SP-Stack?

We are running SAP CRM 5.0, BI 7.0, ECC 6.0 and Enterprise Portal 7.0 and upgraded recently to latest SP-Stack 13. We have used JSPM to upgrade these systems to latest SP-Stack. Based on the upgrade document, JSPM upgrade the SP-Stack and apply latest kernel and we do not need to apply kernel seperately.

My questions

Can we still apply latest kernel released after every 3 months without upgrading to next SP-Stack?

What is the best practice for installation of latest kernel?

Is therre any SAP guideline how frequently we should upgrade the systems with latest kernel? (We applied latest SP-Stack and kernel, though recent EW reports shows your SAP kernel is not-upto-date.)

If we apply the latest kernel seperately, will there be any operational inconsistencies for Java applications (IPC/Internet sales...)especially where we have ABAP+Java systems for CRM ISA ?

Suresh