Serve variation

hi guys I'm hoping you can advise me 

I'm a relative beginner trying to move towards intermediate level and in my club today one of the coaches said I something to me I wasn't sure about.

i was asking advice on mastering the reverse sidespin serve on the forehand as it's the only serve I can't perform at all - I have a topspin serve a backspin serve and a standard sidespin serve, the coach did give me pointers but said he felt this approach was wrong and that I should focus on just getting one serve that I was really good at. 

I've always felt that at any level the best success from serving comes from mixing it up so that my opponent can't settle into returning the same serve all the time regardless how well I hit it.  What would your advice be?

Hi Phil,

I think in general you should try to develop about 3 types of serves.  Then within those serves you can think about lots of variation of spin, speed and placement.  That will give you lots of options to hit your opponent with.