You always play the best kids available if there is no doubt about who is best. If an upperclassman and a lowerclassman are evaluated to be the same skill level you start out playing the upperclassman because you want them both to play. You start out playing the upperclassman on V and the lowerclassman has eligibility to play on a lower level so they start out lower. This doesn't really cause an disruption in your team chemistry and gives a nod to a kid who has put more time grinding day in and day out in the program. I would hold a meeting telling players that nothing is set in stone and you must perform to be on the field. Then as practices and games start you constantly reevaluate who should play at any position and make changes as necessary.
We play in a very mediocre HS conference and we really don't see a Freshman who has the physical strength to out play an average older kid. Not unheard of but also definitely not a normal everyday thing. If a 7-8 grader is competing for a V spot he better be a very rare bird or he should play lower, get reps, and get bigger and stronger.