It is very safe! I wouldn't buy it personally, because the Joie spin seats don't have a lot of leg room when used rear facing, the harness and inserts can also be tricky, and there are other seats which are similar or better for a cheaper price. Joie are generally a good buy when you want a bargain and you don't mind about some of the features being a little bit worse than other brands because the price difference is worth it.
The Axkid Minikid, Move, or Movekid are worth a look if you can have a fit test done in your car. I don't know how exactly they compare to the Joie seat, but they are very compact, usually more compact than isofix seats.
You could look at Britax Swingfix M Plus, which is a very similar style of seat to the Joie, but has a slightly higher weight limit and is also Swedish Plus tested, but £100 cheaper. Or they have the almost-identical seat Dualfix M Plus which is often on discount for about £50 cheaper than that. This doesn't have the Plus test rating, (Confusingly - the Plus in the name doesn't refer to the Swedish Plus Test) because it can also forward face, but with the Britax ones, they are the exact same seat, the Swingfix just has a block in it so it can't be turned to face the front. So the Dualfix is just as safe as long as you only ever use it rear facing. It looks like the Swingfix might have a similar discounted price at some retailers, but the Dualfix is more easily found cheaply.
With the Joie seats, the models which do 180 degree spin (like the spin safe) and the 360 degree spin are not identical - Joie had to add the extra bump in the headrest and do something else to get it to pass the Plus test. So while Joie seats are usually good budget options, and rear facing is very safe regardless, you do get a slight step down in safety if you swap to the Joie i-Spin 360, even if you only ever use it rear facing.
Cybex Sirona Gi i-size might also work, this is similar to the Dualfix in price and spec. It does not have a Swedish Plus Tested version, though.
Having looked for photos of ERF seats in Fiestas:
Axkid One2 seems to fit - this is pricier than Joie Spin safe and no spin, but will last longer (up to 23kg/125cm) and the sliding feature is great, as this gives you very minute control over the child's leg room and front seat space which means you can maximise every centimetre. You can also slide the seat right back when your child isn't in it if you need to carry a taller passenger.
Axkid Move even on full recline, fits with decent leg space in front. Still sold, budget friendly, but won't fit a 6mo. Minimum weight 9kg, but really child should be 80cm+ tall, usually over 12 months.
Minikid 2 - looks to take up about the same amount of space as an old Maxi Cosi Cabriofix on isofix base. This one is not quite as big on the lowest setting as the Move, but you will still struggle to fit a 6mo in it well. It's better from about 8-10 months.
Cybex Sirona was mentioned a few times, as was Britax Dualfix M Plus.
Haven't seen the newer Minikid (4) or Movekid but I think that is just because these are newer. If you can find an Axkid stockist near you, they might be worth a try. They can be used perfectly fine from 6mo on.
Also Avionaut Sky has been suggested as a very compact one. Does not last quite as long as the Axkids, but fine for a 60th centile child. Suitable from birth.
Remember that for a 6 month old, you'll need a lot of these seats more reclined. If you can get away with keeping them in the infant carrier until 12m+, preferably 18m, you'll have a bit more wiggle room because you can install the seats at a more upright angle.