Khay_Cee_Padua
Yes and no? to much xover imo can easily make it confusing, your sister novel have ton of xover but in omake making it less chance to disturb the main story. Soo either make it chat expy, like random adventure from east names okita souji from fgo, or even if you want to summon servant it should be researched first their personality, power, history and background, and see if they can be summoned and have "compatibility" with story. sorry can't really explain it right. But i prefer you just make expy character based on what xover that you want