I would lean towards the GT program inside a school, particularly for your DD8 since they are more likely to have 3rd grade as the main entry point when many GT programs take off. If your DD8 is an older 2nd grader with an early Fall birthday, she would have the opportunity to test in with all the other kids. If she is a 3rd grader, she would not have missed as much of the program, both the academic & social opportunities.
Assuming the GT program is strong/selective, it may actually provide a comparable peer group along with the range of non-academic activities. On the other hand, I wouldn't worry too much about your description of a "hothouse environment". I have found that kids develop strong social and coping skills based more on their innate abilities than the environment, unless said environment happens to be extremely deficient.
