If the stall speed of a plane is 15MPH, does the plane have to reach 15 mph to take off without stalling, or if the thrust is sufficient, will it take off going slower?

Depends on the plane and how much power it has but it seems most model planes are way over powered compared to full scale planes and will fly below stall speed by hanging on the prop. It will be much more challenging to control the plane though, that's why you don't learn to fly with a 3D plane

Technically, and this is from what I've read on full scale planes, planes can take off below stall speed, I think mostly because of flying in ground effect. But if you manage to get out of that ground effect, get the trash bag. (Well, if it's your model plane.)

Full scale planes may get off the ground below stall speed due to ground effect, but pilots don't do it for obvious reasons. The initial goal is to build ground speed way above stall speed. You want to gain altitude as quickly as possible to to avoid any issues. Its safer to gain plenty of altitude and fly current pattern to return to runway than than fly the height of the wingspan(ground effect) and risk crashing into buildings/other traffic.

Yeah, I didn't mean you'd ever want to do that, just that it can be done. The results aren't that great, though!