Hi!

In this game you don't claim a planet until you settle there. But in this game there's also a neat tactic you can use: let one or two races take small and/or for you not colonizable planets in "your" space, but while they'll be researching expensive extreme environments for those planets, you'll research planetary invasion branch. By invading planets they planted in your space, you'll not get only planets, but also those expensive techs as a by-product of invasions. Works best if you have lots of pop.

BR, Iztok

Just agreeing with what the others have said here: don't worry too much about other races grabbing low quality planets deep inside your main area of influence.

After setting up the main high-PQ planets in my initial empire, I'll try to colonize the low-PQ planets out on the "influence borders" first, before I worry about the ones deep inside my areas of influence. If another race grabs those, I can either flip them with influence, or else take them by force. They're easy to invade because they're close to my high-population planets, and I don't have to send troop ships all the way across the map. As others noted, this can lead to some nice techs as a side benefit.

By the same token, I try to avoid colonizing nice juicy high-PQ planets deep inside another race's zone of influence, even if I can reach them during the colony rush ahead of anyone else. This is especially tempting if I have an extreme colonization tech the other race doesn't have, that makes that planet available... but I still try to avoid it. Unless I invest heavily in influence upgrades and nearby influence starbases, I might not be able to keep that planet from flipping to the enemy.