Can I apply to college (full-time) before I ETS?

As early as the University accepts applications.

Back in the day, to get an early out (EM) 90 days, to go to college, I had to be accepted to the school.

I applied before I ETSed. The school doesn't care, nor should the Army (as long as you apply on personal time of course). You should tell the school of your planned start date and meet the deadlines for that date.