Not a twin mum, but do know about car seats.
Personally I would not recommend the Maxi Cosi Mica seat to use from newborn onwards, the reason is that the newborn insert is a one-piece insert, and it's also very bulky. So while it does give a nice position for a newborn, around ~4 months when you need to take the insert out, the seat is much too big and open and doesn't support the baby well at all. Maxi Cosi don't sell an in-between insert either and IME babies don't look right in the seat until they are more like 9+ months (which could be 10-12 months if your twins are small for their age.)
If you're wanting a "0-4 years" (bearing in mind this is only a guide age) type seat from birth the ones which tend to work well are Besafe izi Twist/Turn B (needs to be "B" version to have newborn insert) - but they are much more expensive. Joie i-Spin 360, and possibly the newer one Joie i-Pivot, can work but they can be tricky to get a good fit for very small newborns, which twins can sometimes be, and IME you have to be fairly confident in how to fit a newborn baby into this seat and you will be standing at the side of the car trying to get it right which can take longer, which might be a problem with two (especially if it's raining or very hot/cold).
I don't know how the seats like Silver Cross Motion All Size fit newborns - I haven't had much experience with them. In general I tend to think it's best to try and cover only 1 or max 2 stages with a car seat, not too many.
There are some seats which are extended rear facing from birth up to around 6/7 years which have good support for newborns, but again they are expensive, plus they don't spin to the side.
I would also consider the issue of what you would do if you arrived somewhere and one twin is sleeping but the other one is awake. IME, babies tend to be happy in car seats when the car is moving, but as soon as the movement stops, they want to get out and will let you know about it. If you have one baby, then you can choose to wait in the car until they finish their sleep or you can transfer them into the pram and quickly start moving and they tend to stay asleep, but that might be more difficult with twins. One benefit of portable seats is that you can bring a still-sleeping baby in the seat without disturbing them, which is handy in the first few months where they sleep a lot during the day.
If you did look at infant car seats, you don't necessarily need to go for such an expensive one as Pebble 360 Pro 2. That's £260 per seat plus over £200 for each base and personally I don't see the benefit in a 360 spin for this stage, or a slide feature. But you can get an infant seat + base combo for less than £200 or if you don't need a base, then you're looking at around £100-140 for an infant seat with good positioning for newborns.
For some cheaper ones I'd look at Maxi Cosi Cabriofix i-size with the Familyfix S base, Cybex Aton B2 (usually sold in set with base), Britax Baby-Safe Core, or Avionaut Cosmo.
Avionaut Cosmo has a great position and is very light but is a bit more expensive than the others mentioned, and doesn't tend to go on offer.
It's worth looking for offers on the other brands, as they do fairly frequently. The Maxi Cosi seat lasts less time than the Cybex, Britax, or Avionaut, but this might not matter if you choose to move to fixed seats before the babies are 75cm anyway (which could be a year or so if they are small-average for their age).
It is worth posting on the Multiples board to see what experiences people had between fixed seats vs carry seats. Some of the convenience of a carry seat might be outweighed by the difficulty in carrying two at once, if you can't use them with your pram.