Ok, I know this may sound silly, but what is the difference between 54mm and 1/35 scale? I have seen some after market items that are listed as 54mm, but it appears they were used on 1/35 scale armor. Are they the same scale? If so, how do you figure it out? Any help is appreciated. Thanks.

Theoretically 54mm is 1/32nd scale. I say theoretically because lots of the figures on the market sold as 54mm are slighter bigger or smaller than a pure 1/32nd...
Ciao

MAJMTC, 54mm is also referred to as 1:32 scale. You can get away with using 1:32 and 1:35 scale figures in the same dio. You can also use 1:35 scale figures with 1:32 scale vehicles. As far as using after-market sets, mixing scales might be a problem. Some 54mm items will fit 1:35 scale armor but others won't. I guess it all depends on what they are.

You can mix and match scales. Be careful when you do with respect to your 'base' scale.

Figures (in real life) are all different sizes and therefore can be in a diorama. Equipment is a static size, an M1 rifle for me (6'2" tall) is the same size as for my cousin (5' 4" tall).
You need to keep the equipment all on the same scale.

can't see why you shouldn't mix them as everybody's not all the same size or width so one or two in a dio would be ok :-)