I think alot has to do with special abilities. I'm pretty sure front row for example all have to have scrummaging. As far as attributes, I think they need to be in the mid to high 80s at least. I'll list the attributes for each position that affects their overall rating, not sure how that translates to the chemistry but we'll see
Prop: Scrum, strength, tackle, ruck
Hooker: Hooking, Scrum, strength, tackle, ruck
Lock: Lineout, Scrum, strength, tackle, ruck
Back Row: Handling, tackle, ruck
9: passing, handling, tackle
10: kicking, passing, handling
Center: handling, passing
Wing: speed, acceleration, agility
Fullback: speed, acceleration, handling, kicking
Not sure though, because for example I updated Fourie du Preez and Morne Steyn, both of them got their important attributes in they high 80s or 90s with plenty of special abilities and they still don't have chemistry so I dunno.