Is it possibile, given a sequence of bytes x86 instructions into a stream of random bytes, to decode their instructions?
Yes. Many kinds of processors do it. It is one of the easiest task they have.
Are opcodes of a fixed length
No.
or is there any way to detect those instructions?
The first byte(s) of the instruction allow to infer its length. You will find tables easily on the Internet.
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solved Finding Opcodes by length or something else