Still necessary to tip of there's a service fee?

No, a service fee is often NOT a tip. It is often to pay the staff's salaries. You need to ask the venue though because they are all different. But I would not assume it is a gratuity.

My venue had a service fee of 20% but it did not include tips. We gave each server a flat $40 in separate envelopes for a tip.

You'll have to check with the venue. My 20% fee included tip, but they wouldn't tell me how it was allocated to the staff, so I ended up tipping each server and bartender individually too.

Yes you need to tip with a service fee.

The venue I had and the one I am looking at (see hurricane venue issues) both had service fees and gratuity laid out in their documentation. They are separate, but mine noted both.

Well, I mean I say check your contracts. In some instances service fees are gratuity at my place of work, my gratuity is a service fee.