If it says it takes the Rock then it's a safe bet the Pebble 360 will also be compatible. The only thing might be bulk/weight but the Mountain Buggy is solid enough that shouldn't be an issue.
I would just point this out though as this is a very common marketing line but I think it's a bit of a con! And I think it is useful to understand this before you go to purchase.
later down the line you can upgrade to a rear-facing Maxi-Cosi Pearl without having to buy a new isofix base.
First of all, the majority of toddler seats on the market don't need a separate base. You either fit them with seatbelt and this is OK because they are left in place, or the isofix comes built into the seat. The idea of a detachable base is only really useful for infant carrier seats that you take in and out. And if you have another baby, you'll probably want the base to be available for the infant seat. The toddler seats that go into the modular sets can't be fitted without the base.
Secondly, it's not any cheaper. The Pearl 360 is RRP £299. It is currently on offer for £210, but this is a rare Black Friday offer - it usually sells at around £250/260. The chances of it being on offer exactly when you need it are fairly slim.
Whereas - for less than £299 you can typically buy any of 2-3 spin seat models which are equivalent in features to the Pearl 360, depending on current offers - Graco Turn2Me i-size is consistently £150, safety rating is just as good, though is not as nice in terms of quality as Maxi-Cosi, and then you have Joie i-Spin 360, Maxi Cosi Mica Pro Eco, Britax Dualfix i-Size and Cybex Sirona Gi i-Size which frequently go on offer around £200-250 and are all fairly similar (to each other and to the Maxi Cosi Pearl 360). At any given time, you'll typically find a couple of models in this price range, and you'll have the luxury of choice as can try the different models in your car etc.
Also, right now, today, for around £299 you could get a couple of different spin type seats which have better features than the Maxi Cosi Pearl 360 - for example, Britax Dualfix Plus, which has a higher weight limit of 20kg as opposed to 17.5kg, meaning it will last approximately 1 year longer (depending on child size) RRP £325 but is frequently on offer around £220-250, or Besafe izi Turn, which has a built in angle adjustment to compensate for sloped rear seats, which can be a problem in many rear facing isofix car seats. This one is currently £350 and is usually higher priced.
Or you can get the Silver Cross Motion All-Size 360, which is a spin seat that transforms into a booster seat up to 12 years, with a reasonable safety rating - most all-stages seats don't have this - for £350 (RRP £399)
You can also usually buy a longer-lasting Swedish style extended rear facing car seat for around £300 (e.g. currently Axkid Move is cheaper, usual RRP £280, Axkid Minikid 2 and Britax Safe-Way M are around this price though) - these last much longer, up to approx age 7 as opposed to age 4 for an average sized child. If you have a very tall child, the "to age 4" seats last until age 2-3, which is too young to booster, whereas the "to age 7" seats last until about age 5. These also have much better leg room allowing you to comfortably rear face longer which enables the best possible safety. They are usually also Swedish plus tested which is the highest safety certificate in the world.
Lastly consider the fact that the Pearl 360 is already a couple of years old as a model (2021), so by the time you're upgrading it will be a 3-4 year old model. There is nothing wrong with a 3-4 year old car seat model, especially when it's top of the range like MC Pearl 360 was when it was new, but in the last 4 years of car seat development (for example) we've seen just in the toddler stage:
- Much longer capacity for extended rear facing (higher weight limits)
- Seats which can be used from a younger age (approx 6 months rather than 9-12 months)
- More higher-weight capacity in the spin seat category
- Better side impact protection
- More attention paid to child leg room rear facing
- More choice in the modular, rotating systems
- Better results in (some of) the all-stages car seats, which have previously been poor
- Better results in the purely forward facing, harness+booster category
- A car seat with built in airbags to improve FF safety (and Maxi Cosi have previously done this, which makes me think they might look into it again.)
- Focus on easier installation/error reduction
- New materials being used which allows for longer lifespan/lighter seats/better energy absorption.
None of these are a make or break deal, but it is nice to have the option.