You can be lenient on the temps/gravity/etc on the homeworld, because the race will use those numbers as its baseline. So the world will be a 0.0 cost
Of course, this also means that your new race's habitable range will be determined by the planet you choose as their home world, said raves being centered on the values of that world. So if you pick a how world you'll have more trouble colonising cold worlds - you might want to use SM mode/race to adjust the atmosphere to get the conditions you want.
The biggest impact is probably gravity, as that's unchangable and you can't put people on anything outside of your range. Lower homeworld gravity can open alot of smaller bodies to colonization, for example, but restricts alot of larger ones.