Will low population / new player servers increase my chances of getting the Gurubashi heirloom? I thought the connecting realms meant that there was always going to be the same amount of players regardless.
Generally, because of CRZ, which realm you are on doesn’t really matter. It will be the random luck of the draw which shard you end up in. In a zone where the population is lower than the “cap” for a shard, there will be a Warmode and non-Warmode shard for each data center. (For North America, that’s basically Chicago and LA.)
RP realms do not auto-CRZ in older content, so they can be quieter. However, it’s important to note that is a pretty well known fact, so people will use groups to get themselves onto those realms for special events like the Fishing Contest and Gurubashi Arena. (So, you run the risk of finding highly motivated players on such realms.)
To add on to this, in the most recent set of server merges they connected a bunch of the dead RP servers together.