Graco Slimfit isn't actually that slim either, as it's a multi-group one. So if somebody has fitted 2x Slimfit with a third seat then you potentially have space to do what you like :) But not every combination of 3 random seats will fit, so it's worth checking.
You want belt fitted usually, because isofix isn't flexible enough in fit and you want single stage because these tend to be slimmer. Forget spin because even if you can fit them in, you won't be able to use the spin function so it's pointless. Not so bad if forward facing but the Joie spin is a pain to use RF without the spin. However, it is helpful to try and use what you have so you could try one with 2 other seats to see how you get on.
Check your manual to see if you can put a seat with support leg in the centre seat.
Other than this it is basically going to a car seat warehouse or independent ERF type specialist and doing trial and error, because the width of the seats for example is not as important as the shape of them and how that interacts with the other 2 seats, with the position of the seatbelts, with the angle of the seats that you have.
The 4yo could potentially go into a Swedish rear facing style seat, (Axkid Move, Britax Max Way, Avionaut Sky etc) which would last another year or so until the baby is out of the infant carrier and then they could go into a high backed booster.
Or you could look at slimmer high backed boosters now - Britax, Maxi Cosi and Joie tend to be slimmer than most. The benefit of HBB seats is they don't need a support leg. Isofix is also optional in all of them, even the newer ones.
Or if you want forward facing with a harness (or if 4yo is still under 15kg) you'll be looking at 123 seats, but these tend to be a bit wider plus you have the isofix issue. But this would be your one isofix seat, I would say, if you have to do one isofix. Look for something with a low base so that whatever else you put next to it can potentially rise above it.
The 2yo should almost definitely go into a Swedish rear facing type seat - they are brilliant for 3 across because with the combo of tethers and seatbelt you can fit them flexibly without compromising on the sturdiness. However if you desperately want forward facing for them, I'd look at something like Maxi Cosi Priori or Joei Steadi R129 - these are slimmer belt fitted options with no support leg. Maxi Cosi Beryl is a belt fitted/isofix option with support leg. Possibly good for the centre.
Newborn will be easiest if you can belt in one side seat - but you could potentially put a belt fitted base in the centre seat, if it allows a support leg. That works OK because you can lean over/around the other child seat to click the baby's seat onto or off the base and means you're not fiddling with any seatbelt or harness in the middle seat. Be aware of the combination of seatbelt buckles in the back - you don't want an enthusiastic 4/5/6yo undoing the centre car seat instead of their own seatbelt. It might be worth getting a seatbelt lock that you have to open by pushing your car key into it if you end up with this setup (to protect the belt fitted car seat from accidental unplug)
This is a useful resource with some ideas/seat combos (some quite old so the seat models may no longer be available unfortunately).
erfmission.com/car-seats-3-across-in-the-backseat/