I actually think it would be easier to pick the top five Celtics, as most of their success came with one set of players. Here's my team:
FIRST TEAM
Center, Bill Russell
Power forward, Kevin McHale
Small forward, Larry Bird
Shooting guard, John Havlicek
Point guard, Bob Cousy
SECOND TEAM
Center, Dave Cowens
Power forward, Don Nelson
Small forward, ?
Shooting guard, Bill Sharman
Point guard, Jo Jo White
THIRD TEAM
Center, Robert Parrish
Power forward, ?
Small forward, ?
Shooting guard, KC Jones
Point guard, Dennis Johnson
I need help with the forwards!
Center, Bill Russell v. Shaquille O'Neal
Power forward, Kevin McHale v. Wilt Chamberlain
Small forward, Larry Bird v. James Worthy
Shooting guard, John Havlicek v. Jerry West
Point guard, Bob Cousy v. Magic Johnson
Lakers in 4!
I agree with your first 5 Celtics. But we diverge from there...
First off Tommie Heinson was heads and shoulders ahead of Don Nelson (who was a small forward, I believe). Sam Jones was nearly equal to Sharman at shooting guard. KC Jones was stricly a point guard (he scored in single figures many years)and was superior to Jo Jo White.
Another power forward that is among the top three(who only played a few years with the Celts) is Paul Silas. Without him they would not have won any titles in the mid '70s. Another good small forward was Cedric Maxwell.
Also, while I am a huge Dave Cowens fan, he only had about 5 or six good years. Parrish was excellent for 10 or 12.