I've nothing to say about others receiving the same treatment, but aside from being Pokefied, the design of Oichi is generally the same as it is in Sengoku Musou 3. It might be the same for the the other crossover characters too.
The qualifier should denote which version the character is from. It doesn't matter if the design if similar, I feel the art style is clearly different and should warrant it's own qualifier; when you use the tag as is, you'll get oichi images from two different games, which defeats the purpose of a qualifier.
Same for using the sengoku_musou tag on images for this game; if the artwork or image isn't based directly on the sengoku_musou series, then it shouldn't have the tag.
To be more accurate, Oichi is also a character in the nobunaga_no_yabou game, which is what the crossover game with pokemon is in this case. So if anything, her tag should be oichi_(nobunaga_no_yabou) and only the nobunaga_no_yabou tag applied since that's the crossover game and not sengoku_musou.
It is the same historical character being used between different games, meaning each game is borrowing from history, not each other, and using the different qualifiers reflect that.