l currently have 15/45/195 tyres on my peugeot 205 td, but when people are in the back, they start to rub with the top of the arch, it seems that they are under, but the inside of the arch is rubbing with the side wall of the tyre. its only millimetres that its outside the arch, what are the options l have to try and make it fit under the arch, or to stop it rubbing.

The standard & recommended tyres on that car r 13 inch wheels, with a tyre height of 80% of the width(155 or 165 cm.) = 12.4 cm. that means a total height of 21 inches..
Your tyre should safely be 10.5 inches from the exact center.

NOW YOU HAVE 15 inch wheels, with a sidewall height of 45% of 195cm width. (3 inch sidewalls) That is also a tyre height of 21 inches.

The rear shock absorbers on this car (discontinued in 1998) r very small. Only about 10 to 12 inches long. They r probably worn out by now. have u EVER replaced them?

You cannot add springs, struts, aftermarket racing shocks to this little puff-mobile. (Ha ha. l own 3 of them , '84, '90, '93).
l recommend buying brand new rear shocks. And choose lighter friends.
Even with my two CHILDREN, or if l buy a sack of Cement, l too, notice the rear of the car is lower & handles differently. The 205 is not really a car for 4 adults.
The old 305 (1976 - 1982) had the same engine, but a bigger frame with rear oil filled shocks + coil springs, 24 inches tall. THAT was a 4-5 passenger car. Possibly the later 309 as well.