There are some tricks you can play called "forced perspective" by mixing different scales in a diorama. But, most diorama's I see are constructed in the same, or very close, scale. Following are some sites to check out for figures and other diorama stuff.
Do a search on www.greatmodels.com or www.squardron.com for "1/48 figures". You will find several options. Also, here's another site that has a lot of figures and diorama stuf".
http://www.phpshopxml.com/quakit.shop/CID/50d385b6d9d2863c9feab93eef88ee81/function/controlPageDisplay
Good luck --- post pics....