Jamie.........first of all - you don't need to "protect" an alkyd paint with poly. Decorative paint isn't like auto paint - base coat/clear coat. The durability of decorative house paints is built in - putting a clear over it won't do anything helpful.
As for the yellowing - all alkyd paints yellow to some degree - even the ones that claim to be "non-yellowing" will yellow.
The resin in alkyd paint is golden - it looks like honey - alkyds cure by oxidation (exposure to air) - this oxidation makes the resin more yellow.
Since you are using a cream color - and not white - you likely won't notice the yellowing of your paint.
Your only option to not have this yellow - is to use a latex paint - they don't yellow