I have applied for STD due to Epilepsy but my insurance company is taking too long to approve. Should I am concerned and apply for other benefits such as EI sickness or CPP disability? Would I be eligible for those as I am still an employee at my company but on leave?

What do you mean by too long? They must give you an answer. You should give them a deadline if it's been a month or two, failing which you should advise them that you would be looking at legal action if they don't approve you. You can certainly apply for EI Sick Benefits and/or CPPD, although STD and LTD usually get credit for these benefits. If you are not approved for STD despite being disabled and your doctors confirming your disability, let me know ASAP and we will help. My contact info is below.