I will be on medicare for the full 2018 year but my wife will be on Obamacare for Jan and Feb and then go on medicare. Are the income rules the same for such a short period of time for the Obamacare coverage?
ObamaCare's income rules for assistance always consider your household income for the year, not what coverage you qualify for in a given month. So for example, if you qualify for marketplace coverage for only one month and Medicare only for 11 months, you can still get cost assistance based on your projected household income for the year for that one month (in each month of the year, you are taking 1/12th your total credits, you'll account for this on form 8962 at tax time).
You will however have to make sure your coverage is cancelled when Medicare starts. Generally speaking you cannot hold a private non-group non-Medicare plan and Medicare at the same time.
For the record, this is true for employer coverage and other coverage types like Medicaid too.