Would all of the particular species acquire legs? What if a particular species were separated by great distances? If they were in close proximity to one another, would genetic drift affect all of them at the same time?
They could eventually through interbreeding.
What if a particular species were separated by great distances?
That would be a barrier to interbreeding.
If they were in close proximity to one another, would genetic drift affect all of them at the same time?
Genetic drift is not an "effect", it's a result. Nor is it something that can be coherently described as happening "at the same time". It's what happens when differential reproductive sampling occurs across generations.
"Would all of the particular species acquire legs?
Because the original allele is recessive, as drift causes more and more of the population to include that particular allele in their genome, individuals within the population will occasionally 'pop' up wearing a brand new set of 'legs'. (the legs may just be modification or slight addition). The 'legs' would occur in roughly 25% of the offspring of parents where each parent had at least one 'leg' allele.
"What if a particular species were separated by great distances? "
If they were unable to breed with the 'mutated' population they would be considered a different species. As long as there is very little or no gene flow between the two populations the mutation would affect only the one population. However this really depends on the amount of gene transfer between the populations.
"If they were in close proximity to one another, would genetic drift affect all of them at the same time?
Genetic drift depends on quite a number of factors, but the gene flow between populations, and the population size are very important to the speed and success of the allele fixing in a population. A small population with little injection of alleles from an outside population is the best scenario for drift to occur. As soon as a few critters are born with both recessive alleles and they start having their own little critters, selection takes effect and determines the survivability of the mutation.