Well, it rather depends on what you mean by "uniformly distributed". If you mean that, over all possible lists, each byte position was from a uniform distribution, that doesn't actually tell you that.
Here's the problem: all "uniformly distributed" in this case means is that, in a sufficiently large sample, each value occurs approximately equally often (in the uint_8 case, about 1/256 of the time).
However, it says nothing about correlations, that is, which values tend to immediately follow other values. It is easy to design a 'random string' that is perfectly uniform, however the adjacent bytes are strongly biased; this means that, when you combine adjacent bytes, each uint_16 will not be equally represented. For example, consider a string where, after a byte value $X$, the next position is also the value $X$ with probability 1/2+1/512, and a value $Y \neq X$ with probability 1/512. This sequence is equidistributed; however if you combine uint_8 values, the resulting sequence is strongly biased.
However, if by "uniformly distributed", you mean that the $N$-byte sequence as a whole was selected uniformly from all possible $256^N$ possible sequences, then yes, uniformness is preserved by combining adjacent elements.
Now, getting down to practice: if the list was generated by a cryptographically secure random number generator (CSRNG), then we can assume that combining adjacent elements is harmless. That's because we assume that the output of a CSRNG cannot be efficiently distinguished from random by any method; if the attacker could distinguish it by combining adjacent bytes, that would mean that we really didn't have a CSRNG in the first place.
On the other hand, if the list was generated by a statistical random number generator (that is, a generator that was designed to generate output that "looks" random, that is, passes some standard statistical tests), then it's a tad trickier. Real life generators can have problems when you combine outputs at the "wrong" size; however it is unlikely that one used in practice would have a problem with combining only 2 or 4 elements. So, in this case, you're probably safe (although you might want to check with the generator to be sure)