There is a certain benefit to "confusing" students; I alluded to the ideas of disequilibrium and the resulting equilibration in an earlier MESE post. More comments about Piaget can be found on this site.
In the context about which you are asking: I think that if you want to introduce multiple notations, then you should, at least, abide by two principles:
Make it very clear which notation you will use and be consistent.
Help students to understand why other notation styles exist.
With regard to the second principle, this could be as simple as the $\div$ sign, but you might also consider an example such as how to denote the cyclic group of order three, e.g., $C_3, \mathbb{Z}_3,$ or $\mathbb{Z}/3\mathbb{Z}$.
Each of these has its own advantages and disadvantages. For example:
$C_3$ is easy to remember because the $C$ can stand for cyclic, but its use is not so widespread.
$\mathbb{Z}_3$ is succinct and shows the similarity with $\mathbb{Z}$, but it can also be read as the $3$-adic integers.
$\mathbb{Z}/3\mathbb{Z}$ emphasizes the quotient group aspect, but is often introduced before quotient groups. (Plus, the notation is a bit more cumbersome than either of the previous two.)
Similar remarks could be made about increasing and nondecreasing, or countable and enumerative.
Your question reminds me of a paradigm-shift in theories about language learning. In the past, many thought that teaching a child two different languages would confuse him or her, and, therefore, be disadvantageous. Today, I believe a more common interpretation is that while this can be confusing, it is the resolution of this confusion that can be cognitively beneficial.
With regard to the mathematics classroom: I don't think that varying the use of notation is the best way to induce the sort of productive struggle that leads to learning. Instead, I think discussing why different mathematicians use different notations (as described above) and helping students practice the "bilingualism" involved in moving between, e.g., set theoretical notation and paragraph style proofs are both better ways to raise students' mathematical maturity.