Subs have a higher temp to ignite them. Not a problem with inline guns shooting 209 primers, but can be a problem with sidelocks shooting percussion caps.
3F of subs will work better in sidelocks as far as reliability is concerns, because of the smaller granules. It also buns a bit cleaner and more complete than 2F.

If you're going to prime a load for easier ignition. You should use real black powder. It has the lowest temp to ignite it. I've never done it, so don't believe it's needed. Keeping everything clean and clear should be enough for caplocks. I like Remington #11 caps for rifles, and Rem #10 for revolver/pistols. Everybody has their own opinion on this, but it's what has given me the best luck. I used to shoot CAS, and couldn't afford misfires.

If you were shooting a flintlock. You'd have to use BP for reliability. That shows you how close to failure you are with subs in a caplock. It will work, but you have to make sure everything is right.