Car Seat for 3 different cars, one of which doesn't have Isofix - HELP!

[lalalemon]

The cars we use are:
2015 Ford Focus
2015 Ford Fiesta
2000 Toyota Previa

I've read that Ford are notorious for short seat belts, and I already know the space is not big, we struggle to get the cabriofix in half the time! So not sure what ERF would fit? Especially as it will have to be seatbelt fitted as need non isofix!
Are we going to have to go forward facing? If so, which is the safest forward facing option!

This may be a stupid quesiton, but can you get a car seat which has the option of either being fitted with ISOfix, or with a seatbelt? This way you can move it between cars with different systems. I can't find the answer on google so any help much appreciated!

The Maxi Cosi Pebble can be used with or without it so was perfect for dd when she had to go in grandma's old that doesn't have isofix.

Sorry, I should have said - Group 1.
I think the Pebble is 0+ but I get a bit confused!

The Joie Every Stage should fit all three cars, it takes less seatbelt length than the Stages so it should be ok in the Fords.