An object in 1/48 will be too small to fit with a 1/30 figure, generally speaking. There may be some objects that won't look too bad, but a piece of furniture in 1/48 might tend to look like a piece made for a child, in a diorama in 1/30.
If I remember correctly, 1/48 is 40mm as a size, or O gauge, by some manufacturers' measure (O gauge is 1/43 in some cases).
I agree that going up a scale to 1/24 or 1/25 might work better. You can find items in the car model ranges by makers like Revell, AMT, Tamiya and Fujimi, that can be used for other dioramas--tools, etc.
Hope that helps!
Prost!
Brad