You'll want to get a companion as soon as possible. The darker rat in my dp, Bobby, started to decline in health immediately after his brother passed away. In the time it took me to find another male rat, he lost his appetite and seemed lethargic and less willing to come out of his cage. He was nearly two at the time. Then Tyrion, the white rat in my dp, joined him and his health changed almost over night. It was such a relief. He went on to live another year and half and was nearly four when he passed away!
If the rat you have found for adoption is a similar age to your little guy, then one more rat will be fine. Just consider what you want to do further down the line when one of them is inevitably left alone again. If you're getting a young rat, then you'll want two as they tend to be energetic and need another of the same age to play-fight with. Your old guy probably wont be up for that and it might stress him out.
Your rat will be fine waiting out the quarantine period, so long as you spend lots of time with him. If I even have a rat on their own (for example, after surgery or because they're new) I try to get them out at every opportunity. That could mean they're in my sleeve while I'm at my computer, or out on my bed while I'm in my room. If your rat is a shoulder rat, then have him out on your shoulder whenever it's safe.
As for clashing of personalities, it does happen. I've always found it easier to introduce two rats who were previously alone as they're less 'fussy'. They're usually just happy to have the company since they really do need social enrichment. I've also had more success with the carrier method than the neutral territory method. With the CM you can take it really slow and more or less move along at your rats pace. Some rats take a few attempts to introduce successfully, so don't fret if they don't take to one another right away. As for genuine incompatibility; it does, very occasionally, happen. I think I've had it happen only once and I was then in a position of having to have two separate groups of rats. I'd say it's more likely to go well than wrong though, especially since both rats will be sick of being alone.
Good luck, and do keep us updated.