Is it possible to mix ground start trunks and stations on a analog card?

99% sure the answer is Yes. The analog card hosts 16 "baby boards" and you can mix and match as needed with few exceptions. (Caller ID b-board must be the next circuit to the associated line/trunk b-board.)

I've only used loop start but the same b-board does loop and ground.

Yes, no problem to mix gs trunks and analog station baby boards on a card.