Ensigns should probably be exempt from Recruit Cost Modifiers, such as the basic preferences ones, as its EXTREMELY easy but tedious to sell off all your ensigns, convert their Reqs to Credits, and recruit a new staff, gaining 5 Credits in their career per person, repeating until you can hire any crew you want with your preferences activated
Technically you could do this with higher rank Reqs into Credits as well, but that ends up just being a small bonus for giving up any preferences rather than an abusable loop