Again, I'm not an expert, but this is recent enough in our family's past that I think I'm on solid ground here...
2-word sentences should usually be expressing what the child wants or is doing. "Me milk" or "hold doll" would count. (Pronoun confusion is normal, too.)
At two, I'm pretty sure the "words" don't even really have to be recognizable words to anyone but family. As long as the child uses the same set of sounds consistently applied to an object, that "counts" as a word at age 2. (By 3yo, I believe peditricians would like more than 50% to be recognizable as actual words to other people, as well as the use of 3-word sentences.)
You probably already know this, but just in case, I'll point out that some of that language stuff with a 2yo is just the rate at which the child's throat and larynx develop. Because the physical changes necessary to produce language may come later in some kids, speech may come later for reasons that have nothing to do with intelligence and that will not be helped by speech therapy.
I have a friend with a just-2yo who is HG+ or I'll eat my hat. He's a whirling dervish and is clearly very, very smart, but he's saying almost nothing yet. It's clear that he's trying, and he understands virtually everything said around him, but he can't make the sounds correctly yet. Late talking runs in his family, too, so he's just sort of destined to talk late. I predict that in a year, though, he'll be speaking in paragraphs...paragraphs full of big words, even!
I'd say wait the 6 months if the dr. said that's okay to do and there's no reason to expect problems with speeh, like known birth defects. I mean, 6 months is a quarter of a lifetime for a 2yo! A lot can happen! I think it's way too early to worry.
As for my DS4, you do correctly recall that we supposedly have a high ratio of GT kids in our district, so if DS4 is MG or ND, then that might be enough. Fingers crossed! But he won't be with most of these specific kids, and that's a shame. He's just having such a stellar, happy, productive, fun year! They just all like one another so much. The experience is SOOOOOOO much better than anything DS7 ever had in any school! It restores some of my tarnished faith in school, actually. We had such near-universally blah or downright bad experiences up to this point, with one lone, happy exception, that I was feelng pretty depressed about trying again with child #2. Maybe there is hope for better!
Even so, I don't think this class is the norm around here. Not only because of DS7's experiences, wherein GT kids were NOT all over the place, but also because the other classes in this same pre-K are not like this. In fact, some of the moms of kids that DS4 was in pre-K with last year who are in the 4-day-a-week class were talking with those of us with kids in the 5-day class, and the moms of the 4-day kids are all up in arms because of how far ahead the 5-day kids are. They think it's something the teachers are doing or parental hothousing ("She just won't read. I'm so frustrated! So what books did you use?" Um, maybe she's just not ready to read yet. She is just 4, after all!). Some moms were even talking about trying to switch their kids to the 5-day class mid-year! It was kind of crazy! It looked to me like playground competitiveness at its ickiest.
I suspect (total theory, here!) that the classes wound up being somewhat self-selecting for GTness. The high energy, talkative, GT kids were placed by parents in the 5-day class because the kids seemed to crave it (and to give mom a much-needed break!). The more laid-back, not-so-ready for full-time school ND-ish kids were placed in the 4-day class.
Okay, done digressing... Sorry!
