Contra is the account into which a balancing amount in a transaction goes.
For example, if cash is given by the company to a vendor, then:
Cash Account Credit
Vendor Account Debit (Contra Account)
Salvage value is not handled in FA except if not depreciated after a certain while and a residual value is left in the asset account - book value.
When moving from other accounting systems, make the opening FA Asset Value as the existing written down book value of the asset. This is so that any fresh depreciation can commence from where the old accounting system left off.
Maybe if you have the time, after you have sufficient experience in using the FA Fixed Assets module, you are welcome to fill in the wiki at: