If your mixing BP and composites, they will no all light at the same time as composites take longer.
Also, larger composites take more time than smaller composites because of the larger core volume. That is why multiple igniters are sometimes used in the larger composite motor when mixing large and small composite motors.
Usually BP motors will ignite at the same time whether they are large or small. The important thing is to use a relay launch controller when clustering more than a few motors as the current needed is much higher and rely launch controllers do not have the current and voltage losses that regular launch controllers do.
Another trick is use flashbulbs to ignite a short piece of fuse. Flash bulbs require very little current. However, you need to make sure your launch controller is flashbulb or electric match safe.