Re: hello all new here with a few questions :)
I use Opti-2 and am happy with it at 100-1. A BMX caliper brake will work nicely with that. If you can find one off a donor, do that and use good pads. If you have to buy it bikeworldusa has them. Not all coaster brakes are the same. Does it feel like your brake does a pretty good job? If so, that with a caliper brake is adequate unless your going as fast as possible . I'm not saying it is as good as drum brakes or disc brakes, but on a budget it is way better than just the coaster brake alone. Yes, you can use the kits on coaster brake wheel. All of my bikes have been cruisers with coaster brakes. If you have trouble with your install, feel free to ask questions here.
Someday when I grow up I will probably lose interest in toys with wheels, but until then...