Goodness... that's so heartbreaking, I'm so sorry for your loss - and also for the emotions that come with that, including being unsure what to do with Timbit. They're very very quick and despite their small size, we can't always intervene fast enough.
I've done a lot of rat intros over the years. What I can say is at bare minimum, wait until the babies are closer to his size. While a general rule is that rats are more accommodating to babies/young ones, I've found introducing adult rats/older rats is entirely possible if you're patient in the approach. I introduced 3 seniors (including one who had a history of aggression towards other rats) without any problems whatsoever - sometimes it's just the timing.
All of that said, the reality you're going to have to face right now is that this may never work out. Timbit may never be okay with these babies. Ask yourself if you're okay with this? Can you handle keeping rats in 2 separate colonies? My advice would be to keep Timbit and Espresso together. Even if Espresso is okay with the new ones and interested, Timbit is comfortable with her; she is his companion right now. If Espresso is exceptionally friendly, yes, she can have some free-roam time or even cuddle time on you with the little ones - but they have to go home to separate cages for their safety and to alleviate stress on Timbit.
Keep the cages in the same room but out of reach of grabby rat fingers so nobody is pulling each other from other cages. This can get them accustomed to one another's smells and sounds for the next while as the babies grow.
Do all out time in a designated neutral space (bathrooms work great), and separately (baby group at the same time, then put them away and take out the adults or vice versa). Right now I have 6 rats + 3 we're boarding for someone while they're on vaca, and usually I take them out 2-3 at a time to sit with me while I watch TV or dust or make coffee. That's still socializing with 'em.
If / when you decide to retry intros (again, really advise you wait until they're bigger), neutral space + gently clean them each with unscented baby shampoo right before, or dab some vanilla extract at the bases of their tails. This mutes some of their natural oils, though not all of them, and encourages them to sniff each other. The vanilla approach is by far my favourite due to how well it has worked for us (encourages them to smell each other AND it's nice/a reward to clean off? Win/win). And most of all, take it slow. I've had rat intros that worked out in less than 24 hours, and had others that took 2+ months before I could trust them in the same cage.
Also, as previously mentioned, accept that it may never work out and that you may be raising 2 separate groups for the foreseeable future.