I think it is OK to mix up your 1st serves if you have a variety of pace and spin you can hit - if the returner doesn’t know what he will get next, he will be more stressed especially at big moments. But, you have to execute confidently and not have doubt in your mind as 1st serves are a ‘confidence’ shot (as are overheads) - when you are felling good, the % and pace goes up.
I‘m an advocate of ‘being aggressive to big targets’ or ‘being more conservative, but to smaller targets’ whether it is serves, strokes or volleys. So, if I’m trying to serve big on my 1st serve with a flat or slice serve, I might just be thinking ‘left‘ or ‘right’ side of the service box and not try to be too precise on location. If I hit a slower top-slice or rarely kick serve as my 1st serve, I’ll split the box into wide, body/FH side, body/BH side, down-the-T and pick a target - so, precision and careful selection of the target based on the opponent’s weakness matters if you are going to hit slower serves as your 1st serve on some points. I am much more precise in selecting and executing the serve location on 2nd serves also with many to the body/BH location.
If you hit a kick as your 1st serve sometimes, you could try to have higher RHS and get a higher kick than when you hit it as your 2nd serve - that will also make it more effective.