bonus points if I can make that relationship bidirectional, so the SDR column saying “Eve” also makes the AE column populate with “Adam”
Not possible 
Shameless suggestion box plug: Bi-directionally synced formula columns
As for the Tiers, you want something like this as a formula in your “Tier” column:
SwitchIf([Employee Count] > 1000, 1, [Employee Count] > 500, 2, [Employee Count] > 100, 3, 4)
This supports four tiers (can easily add more). Otherwise for only two tiers, something like If([Employee Count] > 1000, 1, 2) is sufficient.