For your first question -
Well, you can polish over decals and stripes but...
There's a good chance that you'll scuff/tear/remove them with the polish - especially if you use an automated polisher rather than your own hands, and also, you'll find that you get some dried compound around the edges of the decals/stripes which is time-consuming, and sometimes impossible to fully remove.

I'd say your best bet would be to polish the paint before you stick or paint anything on top of it.
If the decals don't stick, then you can easily prepare the polished paintwork.
First wash it down with a PH Neutral soap and water, then wipe the area down with a grease/wax remover.
It's probably best to spray the remover on from one of those garden-spray bottles you can get at any dollar store. While it's still wet, wipe it off. You can maybe do this a couple of times if you want to be really sure it's spotless.

Couple of things to bear in mind -
1. Wait until you're certain the paint has cured fully before you prep it for the decals, and
2. Buy a proper Wax/Grease remover from a proper auto store. There is a difference between Home Depot Mineral Spirits and a proper automotive wax/grease remover. It'll cost you more (I used a Sherwin Williams one before painting pinstripes, and it cost me $22), but it's good insurance.
An impure cleaner could lead to adhesion problems with your decals, or worse still - problems with your paint.

Anyway, here's my $0.02.
1. Paint it,
2. Polish it,
3. Prep it,
4. Decal it.

As for your second question - I'll let someone with more experience on bigger vehicles answer up!