I've got a few questions regarding it:

1. Do I have to have a GA account to activate it?

2. If I use one and still have the two slots I didn't use for a month or more after my GA expires, can I get another GA and still adopt/create two pets to fill those slots? In other words, will I get to keep those slots forever even without a GA for a period of time?

Thank you in advance. ^^

I'm pretty sure this is false.

I have 20 pet slots, and once, my GA expired, but once I activated a new one, I could fill up the pet slots still.

if you have 12 pets, however, and disown two, you should still be able to fill up the pet slots.

so therefore, you can never lose the pet slots. n_n

You can't lose the pet slots, but if they're empty when your GA expires you will need another GA before you can fill them.