See the photos above. Both simplified and traditional Chinese are used. Is there a general rule regarding when should one use a mixture of both? Or is it always a bad practice to mix these twos?
For calligraphy, you can ignore the rules for simplified Chinese characters, especially writing in cursive form like 行書 and 草書.
"Simplified Chinese characters" is the product of political decisions. Some simplified characters are just from their variants. Some are from cursive forms with strokes straightened. Some are from invention with phonetic components. Some might be from simplification in Japan. Some might be from lazy invention. Some might be from strange reasons.
Before the mandatory use of "simplified Chinese characters" in mainland China, people are free to use whatever character variants they like. There is no such bipolar division on "simplified" and "traditional'. The division later is mainly for printing materials and formal form. Cursive form do not have such bipolar distinctions.
You might think some in the picture are "simplified characters" but actually this kind of cursive form exists long before the creation of "simplified Chinese characters". It is not the mixture of "simplified characters' and "traditional characters" in calligraphy sense.