First of all, just want to say I'm looking forward to eventually seeing your dio-story as you're obviously putting in a lot of work!
As for scale; 1:32 sized accessories will be way too small for Joes. Basically the first number is "real world" size and the second number is how many divisions have been made in it to get a smaller size.
Look at it this way: let's say you're thinking of getting a model bike for one of your Dreadnok figures and you've decided on a replica of an actual Harley. Imagine you've got that real bike, that represents your first number, the 1. If the scale of the replica is 1:2 that means you'd need 2 replicas end-to-end to be the same length as the 1 real Harley. If the scale is 1:4 then you'd need 4 replicas to be the same length, and so on. So with that in mind, you'd need 18 replicas in 1:18 (GI Joe) scale to be the same length -- replicas in 1:32 scale would obviously be a lot smaller to be the same length end-to-end. It can be written as a fraction too, really. 1:2 scale means it'll be 1/2 of the size of the original thing. 1:18 scale is 1/18 of the original size, 1:32 is 1/32, etc.
So, the bigger the second number, the smaller the copy will be. The reason maps have numbers as big as 1:100,000 is because they're shrinking hundreds of miles into something you can hold in your hand. GI Joes are only 1:18 because you don't have as far to go to reduce a 6-foot person to hand-held size.
I hope this helped! If it has, my mother will be proud as she's a math teacher. If it hasn't helped, I won't tell her.
Good luck in your dio-creating and props/accessories buying!