I'm no medical professional, but speaking as someone who's taken Zoloft before, as well as lots of DXM, I'd say it's not a good idea. I say that because I'm not sure if it's an MAOI, but if it is, then it's a horrible idea. DON'T DO IT. If it's an SSRI, not as bad. Most of the time I took DXM I was on Prozac, and I was fine.
Also, since you've never done DXM before, I wouldn't try to get high tonight if I were you. Try about 100mg to see if you react badly. At that level you won't be at any serious risk, but it's a good idea.
Then, for your first time, I'd say no more than...maybe 250mg and work your way up from there.
As for the next day, you should be able to talk just fine, as long as you didn't dose at like 5:00 in the morning.
And, at least for me, the day after dosing was a wonderful experience.