Hi Terrie -
If you pin too much, the quilt will get too heavy to move and you'll have difficulty getting half of the quilt under the machine to quilt the middle, which is generally the best place to start to avoid puckers. The rule of thumb for pinning is approximately 4" apart, about the width of your hand. You could need more if you have slippery fabrics or some other complication like that. It also can depend on your quilting style, and that will just come with experience. I don't know the size of your quilt, but say you have a quilt that is 80"x80". If you are pinning 4" apart, you would use about 400 pins.
As for quilting design, you are free to do whatever you want. This is your piece of art. The only rules are to put in enough stitching to secure the layers through washing or whatever plans you have for the future of the quilt, and keeping the density fairly even to keep the quilt from getting wonky (unless wonky is what you want -- it's your quilt). I've always disagreed with Cindy Needham's ESS stitching. You want to do SID in all seams that you want to be viewed as seams. You don't want to stitch seams that you don't want to emphasize, like seams where you had to connect your background fabric pieces together around you pattern (hope that makes sense). And if you're doing an all over, like Baptist Fans, then, in my opinion, SID would fight with your all-over texture.
Baptist Fan would look wonderful on a traditional pattern like Churn Dash. It might be a more difficult pattern to try on your first big quilt because traditionally this pattern is quilted from the outside in or from bottom to top, but you have to weigh that with what moves you for this quilt. Cross-hatch would also look wonderful. You could cross-hatch the whole quilt or you could decide to break up the designs and cross-hatch the centers of the churn dashes and do something different in the bars and half-square triangles and something else in the backgrounds. You could alternate designs in every other block. The sky's the limit! Sometimes doing the same thing in every block can get boring while you are doing the quilting, sometimes changing things up can be overwhelming while you are doing the quilting. That depends on your personality. One rule of thumb is that if you are using several different quilting motifs, try to reuse elements in other parts of the quilt to unify things. So, if you have cross-hatching in your blocks, you might want to quilt piano keys in the border to repeat the straight lines. Like you say, it also adds interest to combine straight lines and curves, so that's something to look at. Could be curved quilting to contrast with your straight lines in the piecing. Could be swirls in the block backgrounds to contrast with cross-hatching in the block centers.
It all comes down to -- do what makes you happy. If it makes you happy, it's the perfect design choice. Enjoy the experience. Don't stress over perfection. Whew, sorry this got to be so long!!!