We had our Meyer's before we got our African Grey. For the first year or so, they didn't really get along - as in, our Meyer's, who is a much much better flier than the Grey, would fly around harassing the Grey, flying near enough to bounce off the back of the Grey. So, we resorted to giving them separate out of cage time. Then we had them at a friend's house for a while as our house had to have some repairs made after a water leak, and when in that unfamiliar environment, they got along great. This was over a year ago now, and they still get on. They don't interact much (and we'd be wary if they did, due to the size difference), but they're happily out at the same time with no issues. So it's certainly not impossible, and they're both African species which is nice
If the red belly is anything like our Meyer's, he's likely to want to harass the newcomer, so you'll need to keep them under close supervision. Silly little pois don't realise they're only one fourth the size of a Grey!
Our cockatoo appeared to get along with them both ok, until after a few months when he got used to his surroundings, and now he thinks it's fun to chase the others around. So because if the size difference to the Meyer's, and the fact that he's a much much better flier than the Grey, we're now on separate times out: the girls go out together and the cockatoo goes out alone. Time will tell if this will change in the future or not
In short, regardless of size difference, anyone who considers adding a parrot to their flock must be prepared that they will not get along and that it may be necessary to have the birds out separately. They may get along fine, or they may not - you can't really tell until you get to know the individuals involved.
Also, I strongly advice that you cover the top of both cages whenever one of the birds are out (especially if the cages are in the same room as you usually have the parrots out in). Parrots tend to get rather defensive about their cages and usually dislike having someone sit on top of it (especially if it's a bird they haven't got a close relationship with), and it only takes a split second for them to bite a toe. We learnt this the hard way, I'm sad to say, though we were lucky no permanent harm was done! We've got one playtop and two dome top cages - while I generally prefer dome tops, when we upgrade to new cages they'll most likely be playtops, simply because it takes the hassle out of trying to cover the top of the cages. In the mean time, we've got plastic sheets that we put over the tops whenever the cockatoo is out - they let some light in, but are thick enough to prevent any biting.
Hope this helps!
