Mixing of field gear is common for all nations, all time period. Even richer nations. Especially if the older pieces are fundamentally compatible with the newer gear. Equipment is durable and expensive, so it's no wonder that universally nations seem to be reluctant to throw the stuff out!
The Warsaw Pact and Soviet militaries were not exceptions, but they also did try to conform to standards. Units with more status generally received the newer stuff sooner. Over time soldiers in secondary/reserve units gradually started looking more like primary/favored units.
DDR went through roughly 3 different equipment standards. This is off the top of my head, so the dates are somewhat rough:
1. Leather belt and Y straps with some equipment in Blumentarn. This was VERY early post WW2 into the late 1950s.
2. Gray webbing belt and Y straps with some Blumentarn equipment. 1960s into the early 1970s.
3. Gray webbing belt and Y straps with some Strichtarn equipment. 1965 through the end of the DDR.
4. Graygreen webbing belt and H straps with most Strichtarn equipment (aka UTV). Mid 1980s through end of DDR.
#2 and #3 were very similar more than dissimilar, but there were changes made such as the construction of AK pouches, features on the Y straps, belt buckle clasp, etc. This means that the later the period of use, the more mixing there was between older and newer kit. The earlier, the less and eventually more pure the kit needs to be.
Here are some links to check out: