Everyone, thank you so much for the replies. Well, even though the matter has been forwarded to the training manager, I am trying to evaluate on things that I could have done better.
1. Pre-flight briefing in regards to decision making. A few of the copilots seem to be under the impression that when they are pilot flying, the ultimate responsibility/decisions are up to them. No where in the SOP is this mentioned but I think I have to politely go over this with the copilots before the start of the day. I am thinking of mentioning something along the lines of 'I will consult you for decisions at all times and I encourage you to speak up but the ultimate decision will have to be made by me as mentioned in the SOPs. You are expected to follow my command unless the safety of the aircraft is compromised'. Maybe you gents have a better way of putting it? I am trying to keep a shallow gradient where they follow my lead but at the same time are not afraid to speak up and the atmosphere is the cockpit is friendly and professional.
2. Pre-flight briefing in regards to maintaining VMC. I think I need to ask them during approach briefing that what is their definition/how are they planning to maintain VMC. Also, I plan to ask them about 'What ifs'. If I have covered all this prior to commencing the descent, I think then both the crews are on the same page and makes life much easier especially when things go wrong.
Anyway, thank you once again for the replies.