Can 1/32 Vehicles and 1/35 Figures be mixed?

Yes, they work together just fine.  I do it all the time, The key us to keep  same sized, like items in the same scale.  For example, keep all weapons in 1/35, or all canteens, etc.  Things like helmets or figures can vary in size since people come in different shapes and sizes as well.

Also foreground/ background. I used 1/220 figures in the background on my 1/160 railroad often and effectively.

Also, stated scales are often way off, so you can arrive at your own conclusions by scaling them, By HeavyArtie is right, shorter guys don't carry shorter rifles.

We have the same problem in Auto, as a lot of stuff is 1/25 and a lot of others are 1/24. eg revell is 1/25 but they own the monogram brand, which are mostly 1/24. you basically have to ask a 'does this look right?' question for each part. if it looks right, it is.