Integrating into mainstream is a red herring. If you find an environment in which the child is happy and confident NOW, adjusting to mainstream or anywhere else in later years is not going to be an issue.
You wouldn't choose a primary school according to how closely it mimics secondary school, for example. You'd choose it because you felt it met the needs of your young child. Choose the right primary setting, and by the time your child is 11 he will be well equipped to tackle whatever comes his way. Unless, of course, he is the sort of child for whom secondary school was never going to be good. But in that case, it wouldn't have helped to throw him into such an unsuitable environment at a younger age, when he is even less capable of tolerating it.
If a previously happy, well-adjusted child struggles hugely with a transition to a different setting, it's because the new setting is wrong for him, not because the previous setting failed to prepare him for it.
My older dd was home educated in what you might consider a very radical way, being allowed to choose for herself what, when and how to learn. She'd only opened a couple of workbooks in her life. She had never done handwriting practice or studied spellings or set out to learn number bonds. When she went to school in Year Five, she had adjusted to school ways within a few weeks. She was popular and was not behind the rest of the class academically. By her nature she's the sort of child who could cope with school well, and having had a happy upbringing gave her the confidence to try anything.
On the other hand, I have serious doubts that my younger dd (also home educated) would jump into school easily in Y5. She isn't cut out for primary school, and it isn't cut out for her. No amount of "preparation" would ever make my square peg slide painlessly into a round hole. Sending her to mainstream at the age of four would only have brought the problems on earlier.
Don't fix your eye on the horizon. Forget about that hypothetical seven-year-old you will someday have. Anticipating his needs is a black art; you haven't even met him yet. Look down at the child in front of you. The child you now have is the one you now need to educate. Meeting his needs right now is the way to ensure he will grow into the best possible seven-year-old.