1) retinoids + vitamin C= irritation, redness, inflammation, dryness, To avoid this use them in different routines. Vitamin C in A.M. routine and Retinoids in P.M. routine

2) benzoyl peroxide + retinoids = dryness & mild to severe irritation can occur, to avoid this use them in different routines. Benzoyl Peroxide in morning routine and Retinoids in P.M. routine

3) retinol + salicylic acid = dryness, severe acne conditions, inflammation, Use Salicylic Acid in the morning routine and Retinol in the night skincare routine.

4) retinoids + AHA/BHA = irritation, redness, inflammation. Use them at alternate nights/routines.

5) vitamin c + AHA/BHA = both are pH-dependent to be active. Use Vitamin C in the morning routine and AHA/BHA in the night skincare routine.

6) BHA + benzoyl peroxide = irritation, redness, peeling, dryness. Use them on alternate routines.

? It is not that these ingredients can't be used together and there wouldn't be any side effects using them together!
But if your skin isn't used to, use them with caution and preferably in alternate routines/ days!