The grid spacing you're seeing depends on your level of zoom. If you zoom in you will see those same line spacing, but the time between them will decrease. That doesn't affect anything when it comes sequencing. It's more for reference than anything in my opinion.
If you selected 50ms when setting up your sequence, then that is the timing for the file. You will be able to drag your effect to lengthen or shorten their duration, all in 50ms increments. Or you can drop timing marks within the timing track, but you can't put them any closer than 50ms.
Effects should for the most part play in the preview window the moment you drop them on the grid, or click on another effect and that one should play automatically, looping continuously. You can however click the stop button on the top of your screen while an effect is playing in the preview window and it will stop. Just click on the effect again and it will play. If you click the play button on top, the sequence will start playing.