Most street heroin is like 5%-50% pure, no way is it 100% pure and especially not if it is tar heroin. Seriously DO NOT do this, heroin is so not worth it. This is a terrible idea, there is no reason to risk your life and risk serious addiction.
But if you are going to be stupid and do it anyway, I'd rather you not die, so:
Yes 0.1g could potentially be enough to overdose from. I have seen people OD from injecting 0.1g. We can't tell you a safe dose, since potency varies SO much plus everyone's different, so all you can do is start very very low, like seriously start with 0.01g-0.02g or something the very first time. Then you can better gauge where to go from there. When I had your (extremely low) level of opioid tolerance I could get really high from just 0.02g, but who knows how the potency of whatever dope you are planning on getting compares to what I had. It could be way weaker or it could be even stronger.
If you start with a small amount and are so high you're worried you may not be able to properly inject more, then do not inject more! If you are so high you can't inject properly you shouldn't be doing more heroin anyway! If you feel nothing at all from the first shot you know you can do a bit more the next time, you should generally only have to wait like 15min before deciding whether or not you need more (assuming you didn't miss or something and aren't taking any other drugs). It is imperative to always start with a very low dose whenever injecting heroin for the first time, whenever doing it after a period of not using, or whenever you get a new batch (even from the same dealer).
If you're thinking of injecting heroin, do a ton of research first on proper injection technique, the risks and complications (for example tar heroin destroys your veins), and read stories from addicts about how horrible addiction is and do not for a minute kid yourself and think you're special and will be able to avoid addiction, and then think long and hard about this decision.
ETA: Also what do you mean about your hand swelling up? Why were you planning on injecting into your hand? That is not a good idea. You should not have significant swelling, that is a sign that something is wrong, although many people do get a histamine reaction which can cause hives or some very light swelling. It has nothing to do with whether or not you heat the dope. You seriously need to research proper injection before even considering this.