Also, if I have a Deerling will it change with the seasons? Or do I have to catch separate Deerling if I want all the different seasonal forms?
Deerling changes according to the season, but it only has this option if it is in your party, they wont change if they're in your PC (which doesn't actually help).
So if you capture a Winter Deerling and we move into Summer, it will change if it is in your party.
But basically no, you only have to capture one Deerling and it will change.